1) YIBUM WITH CHAYAVEI LAVIM AND CHAYAVEI KERISUS
QUESTION: The Mishnah lists several types of prohibitions which exempt a woman from Yibum. One of these types is a "prohibition due to Kedushah (sanctity)," which refers to the prohibition against a Kohen Gadol marrying an Almanah (widow). When the brother of the Kohen Gadol dies childless, the Kohen Gadol and the widow may not do Yibum.
The Gemara infers from the Mishnah that the prohibition against doing Yibum applies to both an Almanah from Nisu'in (who was completely married to the Kohen Gadol's brother) and to an Almanah from Erusin (who was only betrothed to his brother). That an Almanah from Nisu'in does not do Yibum is understandable -- the Torah prohibits a Kohen Gadol from marrying an Almanah with both a Mitzvas Aseh and a Lo Ta'aseh, and the Aseh of Yibum cannot override both an Aseh and a Lo Ta'aseh. However, why is an Almanah from Erusin exempt from Yibum? The Torah prohibits her to a Kohen Gadol only with a Lo Ta'aseh, and the Aseh of Yibum should override the Lo Ta'aseh.
The Gemara first answers that the verse of "Yevimto" (Devarim 25:7) teaches that Chayavei Lavim may not do Yibum. The Gemara then asks that perhaps the verse exempts only Chayavei Kerisus, but it does not exempt Chayavei Lavim from Yibum. The Gemara answers that a different phrase teaches that Chayavei Kerisus are exempt from Yibum and Chalitzah: "If the man does not want to marry his Yevamah..." (Devarim 25:7).
The Gemara then asks, "Mah Ra'is?" -- "what do you see [to include Chayavei Kerisus in the second verse, and Chayavei Lavim in the first verse]?" Perhaps Chayavei Lavim are included in the second verse and thus they may do neither Yibum nor Chalitzah, while Chayavei Kerisus are included in the first verse and thus they may do Chalitzah. What determines which phrase refers to Chayavei Lavim and which phrase refers to Chayavei Kerisus?
The Gemara answers that it is logical that the type of prohibition for which ordinary Kidushin can take effect (Chayavei Lavim) needs Chalitzah, while the type of prohibition for which ordinary Kidushin does not take effect (Chayavei Kerisus) does not need Chalitzah.
RASHI explains how the phrase "Yevimto" teaches that Chayavei Lavim are exempt from Yibum but not from Chalitzah. In the verse, "If the man does not want to marry his Yevamah (Yevimto), then his Yevamah (Yevimto) shall go up to the gate to the elders...," the second word "Yevimto" is not necessary. The verse should have said, "If the man does not want to marry his Yevamah (Yevimto), then she shall go up to the gate..."; it would have been obvious that it is the Yevamah who must go up to the gate. It must be that the extra word "Yevimto" teaches that there is case in which the obligation of Chalitzah applies even though the obligation of Yibum does not apply. Since the verse says, "If he does not want to take (Lakachas)," it implies that Chalitzah is done only when it is possible to do Yibum ("to take" her). The word "Yevimto" teaches that even when there is no possibility of "Lakachas," there is still an obligation of Chalitzah. Had the verse omitted the second "Yevimto," one would have thought that Chayavei Lavim are exempt not only from Yibum but also from Chalitzah.
Rashi's explanation is difficult to understand. Why does Rashi say that without the phrase of "Yevimto," one would have thought that Chayavei Lavim are exempt from both Yibum and Chalitzah? The Gemara clearly states that Chayavei Lavim should be obligated to do Yibum because the Aseh of Yibum should override the Lo Ta'aseh of the Lav, and the Gemara searches for a verse that teaches that there nevertheless is no obligation of Yibum. Rashi, however, says that without any such verse, one would have assumed that Chayavei Lavim have no obligation of Yibum and the only reason why a verse is needed is to teach that they do have Chalitzah. What does Rashi mean? (RISHONIM)
(a) The RASHBA, TOSFOS HA'ROSH, and RITVA answer that Rashi's intention is as follows. The Gemara's source that Chayavei Lavim are exempt from Yibum is not from the extra word "Yevimto" at all, even though the Gemara mentions this Derashah. Rather, the source is from the word "Lakachas" in the same verse. The verse says that only when the Yavam does not want to take her ("Lakachas") does the obligation of Chalitzah apply. That word teaches that Yibum may be done only with a woman whom he could marry as a normal wife (and not just through Yibum), such as by doing Yibum with her, divorcing her, and then remarrying her with the normal procedure of Kidushin and Nisu'in. (The Gemara earlier (8b) derives from "u'Lekachah" that if he divorces her after the Yibum he may remarry her, and the Gemara here (according to this explanation of Rashi) adds that this is a criterion for Yibum: the obligation of Yibum applies only when the man would be permitted to remarry her if he were to divorce her after Yibum). The word "Yevimto" which the Gemara cites adds a second point: now that the word "Lakachas" teaches a case in which there is no Yibum (that is, the Derashah which is not articulated by the Gemara), "Yevimto" teaches that there nevertheless is an obligation to do Chalitzah.
Accordingly, the Gemara leaves out part of the Derashah ("Lakachas") which it should have cited. The Gemara cites the Derashah of "Yevimto," which teaches that Chayavei Lavim must do Chalitzah, because it knew that Rava would ask a question from a Beraisa which discusses that obligation of Chalitzah.
Why does Rashi give this explanation and not the simple explanation that "Yevimto" excludes Chayavei Lavim from Yibum (and not that it includes them in Chalitzah), as Rashi himself implies later (84a, DH v'Ha)?
Apparently, Rashi intends to answer the perplexing questions that TOSFOS raises on the Gemara (DH Iy Hachi and DH Mistavra). Tosfos asks how the Gemara can suggest ("Mah Ra'is") that the word "Yevimto" should teach that Chayavei Kerisus are obligated to do Chalitzah but not Yibum, and that the word "Lakachas" should teach that Chayavei Lavim are obligated to do neither Chalitzah nor Yibum? If "Yevimto" teaches nothing about Chayavei Lavim, no other verse teaches that they are exempt from Yibum. Hence, perhaps both Yibum and Chalitzah apply to Chayavei Lavim (because of the principle that whenever Yibum applies, Chalitzah also applies).
According to Rashi's understanding of the Gemara, Tosfos' question is no question. Chayavei Lavim are not obligated to do Yibum because of a different Derashah -- "Lakachas." Accordingly, even if "Yevimto" does not teach that Chayavei Lavim do not do Yibum, another source teaches that they do not do Yibum -- the verse of "Lakachas." (The Rishonim agree that this is the most straightforward explanation of Rashi's words and of the Gemara.)
(b) The interpretation of the Rishonim, however, does not seem consistent with Rashi's words. According to the Rishonim, the word "Lakachas" teaches that when a normal Kidushin cannot take effect when she is not a Yevamah, the woman is exempt from Yibum as well. However, Rashi (end of DH Yesh Lecha) implies that "Lakachas" means that Yibum cannot be done with her, and not that an ordinary Kidushin cannot be done. Rashi writes that the reason why Chayavei Lavim are not included in the category of "Lakachas" is that "they do not have Yibum"; he does not say that it is because they cannot do a normal Kidushin. (See also RABEINU AVRAHAM MIN HA'HAR, cited later in Insights to Yevamos 20:3. Perhaps the Rishonim's text of Rashi did not read, "[Chayavei Lavim are not included in the category of 'Lakachas'] because they do not have Yibum," but rather, "[Chayavei Lavim are not included in the category of 'Lakachas'] and they do not have Yibum.")
Therefore, perhaps another interpretation for the words of Rashi may be suggested. Rashi understands that when the Gemara asks that Chayavei Lavim should be obligated to do Yibum because the Aseh of Yibum should override the Lo Ta'aseh, the Gemara was uncertain whether the rule of "Aseh Docheh Lo Ta'aseh" indeed applies. In the case of Yibum, it is possible to do Chalitzah instead of Yibum and thereby avoid transgressing the Lo Ta'aseh, as Rashi himself writes earlier (6a, DH Ela) and as Rava later suggests (end of 20b). The Gemara is not asking that the Aseh definitely should override the Lo Ta'aseh in the case of an Almanah who falls to a Kohen Gadol for Yibum. Rather, the Gemara is asking that whether or not the rule of "Aseh Docheh Lo Ta'aseh" applies, there will be a problem in this case: if the Aseh does not override the Lo Ta'aseh (because Chalitzah can be done instead of Yibum) and thus there is no Yibum, then why is there Chalitzah? The Gemara (3a) says that whenever there is no obligation of Yibum, there is also no obligation of Chalitzah! On the other hand, if the Aseh does override the Lo Ta'aseh, then both obligations should apply, Yibum and Chalitzah!
The Gemara answers that perhaps the principle of "Aseh Docheh Lo Ta'aseh" does not apply, and the reason why the obligation of Chalitzah applies is the verse of "Yevimto." In response to this answer, the Gemara asks "Mah Ra'is" -- perhaps Chayavei Lavim have neither Yibum (since "Aseh Docheh Lo Ta'aseh" does not apply) nor Chalitzah (since there is no obligation of Yibum), and "Yevimto" teaches that Chayavei Kerisus have Chalitzah.
The Gemara could have answered that in truth the Aseh of Yibum does override the Lo Ta'aseh of Chayavei Lavim, and the word "Yevimto" teaches not only that Chayavei Lavim do Chalitzah, but that they do not do Yibum (as Tosfos explains). However, from the progression of its questions, it is evident that the Gemara prefers to suggest that "Aseh Docheh Lo Ta'aseh" does not apply here and thus no verse is needed to teach that there is no obligation of Yibum.
According to this explanation, however, why does Rashi (DH Tafsi Bah Kidushin) write with regard to Chayavei Lavim that "Karinan Bei 'Lakachas' b'Di'eved" -- Kidushin takes effect b'Di'eved if one marries a woman who is prohibited to him with a Lav? If "Lakachas" means that he is to take her with Yibum, then even b'Di'eved the Yibum should not work if the Aseh does not override the Lo Ta'aseh (because the Gemara (20b) says that whenever Yibum is prohibited, it does not take effect even b'Di'eved). According to the Rishonim's explanation, Rashi means "Kerinan Bei Lakachas" with regard to normal Kidushin and not with regard to Yibum, and thus Rashi's words make sense. According to the present explanation, however, what does Rashi mean? For an answer to this question, and for an explanation of the words of Rashi later (84a) which imply a different approach to Chayavei Lavim, see Insights to Yevamos 20:4 and 84:2. (M. Kornfeld)
2) THE COMPARISON BETWEEN ZIKAH AND KIDUSHIN
OPINIONS: The Gemara differentiates between Chayavei Lavim and Chayavei Kerisus with regard to the obligation to do Chalitzah. Chayavei Lavim must do Chalitzah, and Chayavei Kerisus are exempt. The Gemara says that Chayavei Lavim must do Chalitzah because Kidushin takes effect with such women, while Chayavei Kerisus are exempt from Chalitzah because Kidushin does not take effect with such women.
What does the Gemara mean when it derives the requirement for Chayavei Lavim to do Chalitzah from the fact that Kidushin with Chayavei Lavim takes effect? Is the Gemara expressing merely an indication ("Siman") that Chayavei Lavim must do Chalitzah, or is the Gemara expressing a causative factor ("Sibah") which is the reason why Chayavei Lavim must do Chalitzah? If it is a "Siman," then the Gemara is not saying that Chayavei Lavim have Chalitzah because their Kidushin takes effect. Rather, the Gemara is saying that since Chayavei Lavim are more lenient with regard to the Ishus of Kidushin, it is logical to assume that when the verse gives an additional obligation of Chalitzah, it is referring to Chayavei Lavim.
On the other hand, if it is a "Sibah," then the Gemara means that since Kidushin does not take effect with Chayavei Kerisus, Zikah also cannot take effect with such women, and if there is no Zikah there is no need for Chalitzah. Since Kidushin does take effect with Chayavei Lavim, Zikah also takes effect and thus there is an obligation to do Chalitzah.
The nature of the Gemara's teaching is apparently the subject of a dispute among the Rishonim which has considerable relevance and implications throughout the Masechta.
(a) RASHI and TOSFOS seem to understand that it is only a "Siman." The fact that there is no Kidushin for Chayavei Kerisus does not necessarily mean that there is no Zikah. Rather, the fact that there is no Kidushin merely provides more reason to assume that this particular verse refers to Chayavei Lavim and is less likely to refer Chayavei Kerisus. It may be possible, however, to find a case in which a Chiyuv Kares does have Zikah, and a case in which a Chiyuv Lav does not have Zikah. For example, TOSFOS (2a, DH va'Achos) writes that a woman who falls to Yibum when she is a Nidah should be obligated to do neither Yibum nor Chalitzah, even though Kidushin does take effect with a Nidah. Similarly, Tosfos (11b, DH Tzarasah) considers Sotah and Machzir Gerushaso to be full-fledged "Arayos" (according to one stage of the Gemara's discussion) even though Kidushin takes effect with them (see Insights to Yevamos 11:2).
Likewise, Tosfos (16a, DH Bnei Tzaros, and 18b, DH Shomeres) mentions situations in which Chayavei Kerisus do have Zikah even though Kidushin does not take effect with them. Similarly, Tosfos earlier (9a, DH v'Harei) suggests that Chayavei Lavin should have Zikah even according to Rebbi Akiva who maintains that Kidushin does take effect with Chayavei Lavin.
Tosfos apparently follows the logic he expresses elsewhere. Tosfos (3b, DH Lo Ta'aseh) writes that the Mitzvah of Yibum overrides the prohibition of "Eshes Ach" (and not that "Eshes Ach" is permitted in a situation of Yibum). This means that the prohibition of "Eshes Ach" is removed only at the moment of Yibum and not at the moment the brother dies childless (see Insights to Yevamos 7:1, and Rashi 52a, DH Nasan). According to Tosfos, every Yevamah is forbidden by an Isur Kares (i.e. "Eshes Ach"), and yet Zikah takes effect with her!
(b) The RASHBA (end of 20b), RAMBAN, and other Rishonim clearly explain that because Kidushin does not take effect with Chayavei Kerisus, Zikah also does not take effect and thus there is no need for Chalitzah. The Rashba and others indeed disagree with the assertions of Tosfos mentioned above, because they maintain that when there can be no Kidushin there is no Zikah, and when there can be Kidushin there is Zikah. Similarly, these Rishonim disagree with Tosfos about the status of a Yevamah before Yibum. They maintain that the Isur of "Eshes Ach" is removed at the moment of the brother's death and not at the moment the act of Yibum is performed. (M. Kornfeld)
3) DOES YIBUM DONE UNLAWFULLY CREATE KIDUSHIN "B'DI'EVED"?
QUESTION: The Gemara assumes that when a Lo Ta'aseh prevents a man from doing Yibum with his brother's widow, if the brother transgresses the Lo Ta'aseh and does Yibum with the woman who is prohibited to him, b'Di'eved the Yibum does not take effect and she still needs Chalitzah.
Why does Yibum not take effect b'Di'eved? After all, when one marries a woman who is prohibited to him with a Lo Ta'aseh, the Kidushin takes effect. Why should Yibum not take effect as well, b'Di'eved?
Even if the Torah specifically excludes Chayavei Lavim from the obligation of Yibum (just as it excludes an Aylonis and "Eshes Achiv she'Lo Hayah b'Olamo" from Yibum) and thus it is not possible for him to do an act of Yibum, nevertheless at least Kidushin (Kidushei Bi'ah) should take effect when the brother does Yibum with her (and thus she should need a Get from him). The reason why Kidushin does not take effect with the Yevamah in the cases of Aylonis and "Eshes Achiv she'Lo Hayah b'Olamo" is that these women are prohibited with the Isur of "Eshes Ach." In contrast, in the case of Chayavei Lavim the Torah says that Chalitzah is done, and whenever Chalitzah applies the Isur of "Eshes Ach" is removed (as the Gemara says on 10b; see Insights there). Therefore, when one does Yibum with such a woman there should be no Isur of "Eshes Ach" and the Kidushin should take effect.
ANSWERS: The SHA'AR HA'MELECH (6:11) and AVNEI MILU'IM (174) discuss this question at length. They refer to two primary approaches in the Rishonim as to why Kidushin does not take effect when a man does Yibum with a woman who is prohibited to him with a Lo Ta'aseh. (These two approaches depend on the two approaches mentioned in Insights to Yevamos 20:2, 7:1, and 10:1.)
(a) RASHI (52a, DH Nasan Lah) writes that in a normal case of Yibum, during the period of Zikah the Yevamah is prohibited to the brother because of "Eshes Ach." The Torah gives him a special allowance to do Yibum, at which moment the Mitzvah of Yibum overrides the Isur of "Eshes Ach." Until he does Yibum, however, the Isur remains in force. This explains why one cannot betroth a Yevamah with Kidushin; she is prohibited to him as "Eshes Ach" and the Kidushin cannot take effect.
With regard to Chayavei Lavim, the Torah says that a man may not live with a Yevamah who is prohibited to him because of a Lo Ta'aseh, and thus he has no active obligation to do Yibum with her (the Torah proscribes it). Therefore, the Isur of "Eshes Ach" is not pushed aside for Yibum and the woman remains an "Eshes Ach." Since she is prohibited to him with an Isur Kares of "Eshes Ach," he cannot marry her even with normal Kidushin. In other words, the Lav causes the Isur Kares of "Eshes Ach" not to be removed, and thus Kidushin cannot take effect. (When the Gemara (10b) says that the Isur of "Eshes Ach" is removed by Chalitzah, it means that the Isur is removed only at the moment that Chalitzah is done but not prior to it.)
(b) The RASHBA (end of 41a) writes that the Isur of "Eshes Ach" is removed as soon as the woman falls to Yibum. From that moment onward, none of the other brothers are prohibited to her because of "Eshes Ach." Accordingly, it cannot be the Isur of "Eshes Ach" that prevents the Kidushin from taking effect. However, Kidushin still does not take effect; if the brother is Mekadesh the Yevamah with Kesef or with a Shtar, only a Kidushin d'Rabanan (called "Ma'amar") takes effect, but not a Kidushin d'Oraisa. The Gemara in Kidushin (14a) cites a Beraisa which explains that the verse, "Yevamah Yavo Aleha" -- "Her Yavam shall come upon her" (Devarim 25:5), teaches that Kidushin performed with Kesef and Shtar is not valid when done with a Yevamah. The marital bond of Yibum can be formed by being Mekadesh the Yevamah in only one way -- with Bi'ah, and not in the other two ways (Kesef and Shtar) normally used to effect Kidushin. This explains why Kidushin cannot take effect with Chayavei Lavim -- Kidushin never takes effect in a situation of Yibum.
(The MAHARSHA (20b), however, seems to understand that Kidushin can take effect with a Yevamah. Similarly, RABEINU AVRAHAM MIN HA'HAR (20a) implies that Bi'ah with a Yevamah who is prohibited with a Chiyuv Lav effects Kidushin even though it does not remove the Zikah. In such a case, the woman will be bonded to her Yavam through both Kidushin and Zikah, and thus she will need both Chalitzah and a Get (mid'Oraisa) to break the bond with the Yavam. See SHI'UREI RAV NACHUM (Pertzowitz) who discusses the opinion of Rabeinu Avraham Min ha'Har at length.)
4) DOES YIBUM DONE UNLAWFULLY TAKE EFFECT "B'DI'EVED"?
QUESTION: The Gemara (20a) initially derives from the verse, "Yevimto," that Chayavei Lavim are exempt from Yibum and are obligated to do Chalitzah. This Derashah implies that they are prohibited mid'Oraisa from doing Yibum.
Rava disproves the Gemara's initial assumption. If the Torah prohibits Chayavei Lavim from doing Yibum, then if one transgresses and does Yibum with a woman who is prohibited to him with a Lav, the Yibum should not take effect and the Yavam should not be Koneh her. The Beraisa, however, states that b'Di'eved one who transgresses and does Yibum with such a woman is Koneh her. Rava therefore concludes that the prohibition against doing Yibum with Chayavei Lavim is only mid'Rabanan. The Rabanan prohibited Yibum with Chayavei Lavim lest the Yavam live with the prohibited woman a second time, after he has already fulfilled the Mitzvah of Yibum ("Bi'ah Rishonah Atu Bi'ah Sheniyah."
Rava then rescinds his view and asserts that the Torah does not apply the principle of "Aseh Docheh Lo Ta'aseh" to obligate one to do Yibum with Chayavei Lavim. The principle of "Aseh Docheh Lo Ta'aseh" does not apply in such a case because it is possible to fulfill the Aseh without transgressing the Lo Ta'aseh -- such as by doing Chalitzah instead of Yibum. Whenever it is possible to avoid transgressing the Lo Ta'aseh, the Mitzvas Aseh does not override the Lo Ta'aseh. Rather, it must be that mid'Oraisa there is no obligation of Yibum with Chayavei Lavim.
The Gemara proceeds to refute Rava's revised opinion and to support his original opinion that Chayavei Lavim are obligated mid'Oraisa to do Yibum and it is prohibited only mid'Rabanan. The Gemara quotes the Beraisa which says that if the brother transgresses and does Yibum with the woman, b'Di'eved he is Koneh her and the Yibum is effective. If the Yibum is effective b'Di'eved, it must be that mid'Oraisa there is an obligation to do Yibum (because of "Aseh Docheh Lo Ta'aseh"), and the prohibition against doing Yibum with Chayavei Lavim is only mid'Rabanan.
However, Rava himself relied on this Beraisa when he refuted the opinion that mid'Oraisa there is no Yibum for Chayavei Lavim. How, then, can he now advocate the opposite view which the Beraisa disproves? (TOSFOS DH Meisivei)
(a) The RASHBA points out that the Gemara's way of expressing that Rava changed his mind is, "Rava said -- and some say Rav Ashi said...." These words imply that the Gemara is unsure who said that there is no Yibum for Chayavei Lavim even mid'Oraisa (because the Aseh does not override the Lo Ta'aseh in this case). If it was Rava who quoted the Beraisa earlier to refute the opinion that there is no Yibum mid'Oraisa, then he could not be the one who said that the Aseh does not override the Lo Ta'aseh; it must have been Rav Ashi who ruled this way. If, on the other hand, Rava is the one who said that the Aseh does not override the Lo Ta'aseh, then the discussion until now must have been the words of Rav Ashi and not Rava (even though the Gemara does not mention Rav Ashi's name earlier). The Gemara here means that both statements could not have been said by the same person.
(b) The PNEI YEHOSHUA (in Kuntrus Acharon) suggests a simple answer for why Rava thought that the Aseh does not override the Lo Ta'aseh since it is possible to do Chalitzah and avoid the Lo Ta'aseh, while at the same time Rava was not bothered by the Beraisa which says that the Yibum is effective b'Di'eved:
The Gemara initially understands that the Aseh does not override the Lo Ta'aseh because the verse says "Yevimto," which teaches that Chayavei Lavim do not do Yibum. If the verse itself says that Chayavei Lavim do not do Yibum, then it is obvious that there is no Mitzvah of Yibum even b'Di'eved; the Torah never gave a Mitzvah of Yibum to Chayavei Lavim. Therefore, if the brother does Yibum his act is meaningless (as explained in the beginning of the previous Insight).
However, according to Rava (after he changed his mind), the verse does not relate to Chayavei Lavim and Yibum. Rather, it is logical that Chayavei Lavim do not do Yibum, because the Aseh does not override the Lo Ta'aseh when one can fulfill both the Aseh and the Lo Ta'aseh by doing Chalitzah instead of Yibum. Since the verse does not say that Yibum is not done by Chayavei Lavim, perhaps the Mitzvah of Yibum does apply to Chayavei Lavim, but there is merely an Isur Lav which prevents one from fulfilling the Mitzvah l'Chatchilah. (The laws of Yibum do apply to him, but an Isur Lav prevents Yibum from being done.) Consequently, if he transgresses the Lav and does Yibum, b'Di'eved the Kinyan of Yibum takes effect because the law of Yibum does not exclude Chayavei Lavim.
The Gemara does not accept this argument. It asserts that even according to Rava's reasoning, if Yibum was done it should not be effective b'Di'eved, because the Isur Lav which prevents one from doing Yibum l'Chatchilah should also prevent the Yibum from taking effect b'Di'eved. The Gemara considers it logical that the Torah never meant to include Chayavei Lavim in the law of Yibum if the Lo Ta'aseh prevents one from doing the act. (The Pnei Yehoshua suggests another reason for why the Gemara rejects Rava's view.)
Rava's opinion is now clear. He maintains that the Beraisa which says that Yibum is effective b'Di'eved does not contradict his statement that Yibum with Chayavei Lavim is prohibited mid'Oraisa.
The approach of the Pnei Yehoshua may resolve a difficulty with the words of Rashi elsewhere. REBBI AKIVA EIGER (in Gilyon ha'Shas here) points out that Rashi in a number of places (Yevamos 84a, DH v'Ha Kulei; Sanhedrin 53a, DH Isur Mitzvah) seems to side with the view that mid'Oraisa Chayavei Lavim may not do Yibum, in accordance with the first opinion in the Gemara and as Rava says in his revised opinion. This view of Rashi is implicit in a number of other places as well (9a, DH v'Harei Isur Mitzvah, and in the Mishnah, 20a, DH Gerushah).
Why does Rashi side with that view? The Gemara here clearly concludes that Chayavei Lavim are prohibited from doing Yibum only mid'Rabanan. The Gemara reiterates this ruling in Sanhedrin (19a). Moreover, the Gemara proves that Yibum for Chayavei Lavim cannot be prohibited by the Torah from the Beraisa which states that b'Di'eved, the Yibum is effective. Why does Rashi side with the Havah Amina of the Gemara?
Rashi may understand that although the Gemara here and in Sanhedrin says that the prohibition against Yibum with Chayavei Lavim is only mid'Rabanan, other sources indicate that Chayavei Lavim are prohibited mid'Oraisa from doing Yibum. Therefore, Rashi concludes that this matter is the subject of debate among the Sugyos. The Sugyos which maintain that Yibum with Chayavei Lavim is prohibited even mid'Oraisa follow Rava's conclusion that the Aseh does not override the Lo Ta'aseh (since it is possible to do Chalitzah instead). The Beraisa does not contradict this opinion because it is possible that Yibum is effective b'Di'eved, as the Pnei Yehoshua explains. (According to the explanation mentioned in Insights to Yevamos 20:1, this is also the approach of the Gemara at the end of 20a. Rava does not reject that approach when he cites the Beraisa which says, "Im Ba'alu Kanu.")
What are Rashi's sources that support Rava's conclusion (that Yibum may not be done with Chayavei Lavim even mid'Oraisa)?
1. Rashi (DH Teyuvta) writes that the opinion that Yibum with Chayavei Lavim is prohibited only mid'Rabanan contradicts the view of Reish Lakish who says that when it is possible to fulfill both the Aseh and the Lo Ta'aseh, the Aseh does not override the Lo Ta'aseh. The Acharonim ask why Rashi writes this if the Gemara -- on the very next page (21a) -- reconciles this opinion with the view of Reish Lakish. The Gemara there says that the Chalitzah is not a bona fide fulfillment of the primary Mitzvah of Yibum, and thus it cannot be said to take the place of Yibum. Rashi's text of the Gemara apparently did not include those words, as he writes that the Gemara refutes the view of Reish Lakish. Rashi understands that the Gemara never reconciles the opinion of Reish Lakish with the opinion that, mid'Oraisa, Chayavei Lavim have Yibum.
If the Gemara's conclusion here -- that Yibum for Chayavei Lavim is prohibited only mid'Rabanan -- is correct, one would expect the Gemara to reject the view of Reish Lakish and not quote him again elsewhere. However, the fact that the Gemara quotes his opinion in many places implies that the Gemara elsewhere supports his view (that Yibum for Chayavei Lavim is prohibited mid'Oraisa) and disagrees with the conclusion of the Gemara here (that Yibum for Chayavei Lavim is prohibited only mid'Rabanan).
2. A number of Sugyos in Yevamos support the view that Yibum for Chayavei Lavim is prohibited mid'Oraisa. The Gemara (9a) says that according to Rebbi Akiva who rules that Kidushin does not take effect for Chayavei Lavim, not only do they not have Yibum but they also exempt their Tzaros from Yibum, just as an Ervah does. TOSFOS (9a, DH v'Harei) asks that until now the Gemara has taught that even an Ervah herself should do Yibum if an Aseh can override a Lo Ta'aseh which has Kares. Accordingly, Chayavei Lavim -- which do not have Kares -- should certainly do Yibum because of "Aseh Docheh Lo Ta'aseh," regardless of their status as Ervah according to Rebbi Akiva! (See Insights to Yevamos 20:2 and 12:1.)
Perhaps Rashi infers from there that according to the Gemara's conclusion, the Aseh of Yibum does not override any Lo Ta'aseh, and therefore Chayavei Lavim are treated like any other Ervah.
3. The Gemara (6a) mentions that even if an Aseh overrides a Lo Ta'aseh which has Kares, the Aseh of Yibum does not override such a Lo Ta'aseh because the obligation of Yibum can be discharged through Chalitzah. (This is how Rashi understands the words "Hechsher Mitzvah" there; see Insights to Yevamos 6:1.)
Tosfos there asks that this is not true. The Gemara here (20b-21a) says that Chalitzah is not considered an equal alternative to Yibum; the primary Mitzvah is Yibum, as the Gemara concludes (21a). Nevertheless, the Aseh of Yibum still overrides the Lo Ta'aseh even though one can avoid the Lo Ta'aseh by doing Chalitzah.
Rashi, in contrast, maintains that "Hechsher Mitzvah" means that if one can avoid the Lo Ta'aseh by doing Chalitzah instead of Yibum, the Aseh of Yibum does not override the Lo Ta'aseh. According to Rashi, the Gemara there (6a) accepts the argument of Rava that Chalitzah is not considered an equal alternative to Yibum, and thus Yibum is not comparable to an ordinary case of "Aseh Docheh Lo Ta'aseh." This serves as another source that Yibum is not a case of "Aseh Docheh Lo Ta'aseh."
4. The final source is the Gemara earlier (11b) which teaches a Kal v'Chomer: if a man is prohibited from remarrying the woman he divorced if she married another man (who then died or divorced her), then certainly that man's brother should be prohibited from marrying her, and perhaps even her Tzarah. This Kal v'Chomer is puzzling. A man is prohibited from remarrying his divorced wife with an Isur Lav of "Machzir Gerushaso." How can the Kal v'Chomer teach that the Yavam should not be able to do Yibum? Even if the Yavam is prohibited to the woman with an Isur Lav, the principle of "Aseh Docheh Lo Ta'aseh" should apply and permit him to do Yibum (and certainly the Tzarah should do Yibum, since the Lav does not affect her). (RASHASH)
According to Rashi, however, who says that the Torah prohibits Chayavei Lavim from doing Yibum, the Gemara's logic is clear. Since the woman is prohibited to her husband with a Lav, she is also prohibited to her Yavam with the power of a Lav, and thus there is no Yibum. (Since this Lav has an element of "Ervah," there exists the possibility that even her Tzarah will be exempt from Yibum.)
Based on these sources, Rashi may have concluded that most Sugyos reject the conclusion of the Gemara here and instead accept Rava's argument that the Aseh of Yibum does not override a Lo Ta'aseh, and if Yibum is done nonetheless, b'Di'eved it is effective (because the Torah does not explicitly exclude Chayavei Lavim from Yibum).
(This approach may be taken a step further. Even if the Torah explicitly excludes Chayavei Lavim from Yibum with the verse "Yevimto," presumably the reason why it excludes them is that Yibum cannot override a Lo Ta'aseh since the Aseh can be fulfilled through Chalitzah. If this would not be the reason, then this verse would serve as a source that an Aseh never overrides a Lo Ta'aseh, as the Rashba indeed asks based on the Gemara earlier (6a). It must be that the Torah excludes Chayavei Lavim from Yibum specifically to teach the principle that whenever one is able to fulfill the Aseh without transgressing the Lav, the Aseh does not override the Lav. Hence, the verse does not exclude Chayavei Lavim from Yibum entirely; rather, it teaches that an Aseh which can be fulfilled without transgressing the Lo Ta'aseh does not override a Lo Ta'aseh. Accordingly, if one transgresses and performs Yibum with Chayavei Lavim, the Yibum takes effect b'Di'eved, as the Pnei Yehoshua suggests according to Rava's conclusion. Based on this approach, the intention of the Sugya may be as follows: Rava initially thought that the verse excludes Chayavei Lavim from Yibum entirely. He later changed his mind and suggested that the verse teaches only that an Aseh which can be fulfilled without transgressing the Lav does not override the Lav.)
This also explains the comment of Rashi (end of 20a) in which he writes that the reason why Chalitzah is done with Chayavei Lavim is that "Karinan Bei 'la'Kachas'" -- and since, b'Di'eved, "Lakachas" applies to Chayavei Lavim (that is, Kidushin takes effect b'Di'eved), the obligation to do Chalitzah applies. Why does Rashi write that Yibum with Chayavei Lavim is effective b'Di'eved if the Gemara throughout the next page says that if Yibum is prohibited mid'Oraisa with Chayavei Lavim, it does not take effect even b'Di'eved? (See end of Insights to Yevamos 20:1.)
The answer is that Rashi there follows the opinion of Rava at the end of the Sugya, that the Aseh of Yibum does not override a Lo Ta'aseh since it is possible to do Chalitzah instead (as explained above, Insight 1:b). This explains why Rashi says that b'Di'eved if one did Yibum, it is effective. (M. Kornfeld) (With regard to the intention of Rashi on Daf 84a, see Insights to Yevamos 84:2.)
(It should be noted that Rashi's words in Sanhedrin (53a, as cited above) are particularly difficult to understand. Rashi first writes that the Torah exempts Chayavei Lavim from Yibum. In his very next comment, Rashi explains that although the Torah obligates them to do Yibum, the Rabanan prohibited them from doing Yibum because of "Bi'ah Sheniyah." (See MAHARSHA there who suggests a somewhat forced answer to reconcile the apparent contradiction.) It seems that the two comments of Rashi there reflect two distinct approaches to the Sugya, and that Rashi at one point revised his explanation and added the second approach. Both explanations were included in our edition of Rashi, even though Rashi rejected one of them. Another example of such a phenomenon appears later in Yevamos (23b) -- see Rashi DH Mai Shena and DH Choltzos; see Tosfos and Rishonim there. See also Appendix to Insights to Maseches Eruvin, where this phenomenon in the text of Rashi is discussed.)