1) REBBI CHIYA'S HALACHAH AS APPLIED TO THE CASE OF "BITO"
QUESTION: Rebbi Chiya states that it is possible to apply the following Halachah to all of the cases in the Mishnah (2a): Two sisters married two brothers, and those two brothers died. One of the sisters is an Ervah to one of the remaining brothers (Levi), but is not an Ervah to the other remaining brother (Yehudah), while the other sister is an Ervah to Yehudah but not to Levi. In that case, each sister may do Yibum or Chalitzah with the brother to whom she is permitted, and there is no problem of "Eshes Zekukaso" since the other woman is not considered Zekukah to the man to whom she is forbidden. ("V'Kulan Ani Korei ba'Hen....")
The Gemara says that Rebbi Chiya's Halachah can apply to the Ervah of "Bito" (where each Yevamah is forbidden to one of the brothers because she is his daughter, but she is permitted to the other brother) only when the Yevamah is the brother's daughter through Onsin (rape) and not through Nisu'in (marriage). RASHI (DH b'Onsin) explains that the reason is that it is impossible for two brothers to have been married to the same woman such that they have two daughters who are sisters from the same mother. The second brother cannot marry a woman who was married to the first brother (she is "Eshes Achiv," the wife of his brother who left behind children). Therefore, the only possible case in which two brothers have two daughters from the same woman is when they raped that woman and bore the two daughters.
Rashi explains that the reason why both brothers could not have been married to the mother of their daughters is the Isur of "Eshes Ach." However, that reason does not suffice to explain why one brother could not have been married to her. Perhaps the first brother raped her, and the second brother married her, in which case there is no prohibition of "Eshes Ach"! Rather, there is another reason why the case cannot be one in which one brother raped her and the second brother married her. In such a case, the first brother's daughter (who married a third brother) could not fall to ether brother for Yibum -- the first brother, because she is his daughter, and the second brother, because she is "Bas Ishto" (the daughter of his wife)! It must be that the daughters came from a woman who was raped, and thus "Bas Ishto" does not prohibit one brother to the daughter of the other brother. This reason explains why -- even after the first brother raped the woman -- the second brother also must have raped the woman and could not have been married to her. If the second brother married the woman, there would still be a problem of "Bas Ishto." In contrast, if the only problem is "Eshes Achiv," then there is no reason to say that the second brother raped the woman and was not married to her. (See MAHARSHAL.)
Rashi himself seems to have noted this point. Rashi writes that the second brother also raped the woman (and he does not say that only the first brother raped her, while the second brother married her). Why, then, does Rashi not give the reason of "Bas Ishto" as the reason why it is not possible to have two Yevamos who are the daughters of the two Yevamim through marriage?
This point is made by TOSFOS and TOSFOS HA'ROSH earlier (9b).
ANSWER: It appears that Rashi follows his own opinion as expressed elsewhere. The Gemara later in Yevamos (94b) records a dispute between Rebbi Yishmael and Rebbi Akiva with regard to the prohibition against marrying one's mother-in-law (Chamoso). According to Rava's understanding of the dispute, Rebbi Akiva rules that the punishment for marrying one's mother-in-law (Sereifah) does not apply to one who marries her after the death of his wife. Rebbi Yishmael argues and rules that the punishment still applies in such a case. RASHI in Sanhedrin (76b) explains that according to Rebbi Akiva no Isur Kares or Lo Ta'aseh prohibits one from marrying his wife's mother after the death of his wife. His wife's mother is only "Asur b'Arur" to him -- the Torah places a curse on a man who marries his mother-in-law; no explicit Torah prohibition forbids it.
The Rishonim discuss whether this leniency also applies to the Isur of "Bas Ishto" after the death of one's wife. Is the Isur of "Bas Ishto" comparable to the Isur of "Chamoso" (since both Isurim stem from the same verse (Vayikra 18:17), which prohibits marrying a mother and her daughter), or does this leniency apply specifically to the case of "Chamoso" and to no other Isur? (See MAGID MISHNEH, Hilchos Isurei Bi'ah 2:7.)
Perhaps Rashi maintains that after the death of one's wife, he is no longer prohibited to any of her daughters (who are not his own daughters). That is, the Isur of "Bas Ishto" is removed after the death of one's wife. Accordingly, a case in which two daughters were born to two brothers from the same woman to whom both brothers had been married -- and yet the daughters are not prohibited to both brothers because of "Bas Ishto" -- is easy to find. Such a case exists when Levi married the woman, she bore him a daughter, they were divorced, and after her divorce from Levi she married the second brother, Yehudah, and bore him a daughter -- and then she died. Only Levi's actual daughter is forbidden to him (because of "Bito"), while Yehudah's daughter is not forbidden to Levi as "Bas Ishto" since her mother (his wife) is no longer alive.
Rashi writes that the second brother raped the woman, and not the he was married to her, in order to simplify the case by not involving the death of the woman. Nevertheless, even if the second brother would have been married to the woman, it would have been possible for each brother to be permitted to the daughter of the other brother -- such as when the mother of the girls died (after her marriage to the second brother).
This approach answers another question. Why do Rav Yehudah and Abaye say that Rebbi Chiya's Halachah does not apply to all of the cases in the Mishnah because "he refers only to cases of Nisu'in"? Rashi earlier (9b) explains that they mean that the Mishnah discusses only cases of Nisu'in. This explanation is problematic, because Rashi himself (2a) explains that the Mishnah must be referring to cases of Onsin (rape) since it lists "Bito" and "Bas Ishto" as separate Arayos! If "Bito" means one's daughter through marriage, then that daughter is already forbidden to him because of "Bas Ishto," which is mentioned separately in the Mishnah.
The answer to this question is that according to Rav Yehudah and Abaye, the Mishnah indeed may be referring to cases of Nisu'in as well, where "Bito" was born from one's wife and not from a woman one raped. Nevertheless, "Bito" is not included in the Ervah of "Bas Ishto" because in a case in which one's wife died, the Isur of "Bas Ishto" is removed, yet the Isur of "Bito" remains. (Rashi on the Mishnah (2a), however, explains the Mishnah according to Rebbi Chiya; see Insights to Yevamos 2:1 and 9:3.)
2) REBBI CHIYA'S HALACHAH AS APPLIED TO THE CASE OF "ESHES ACHIV SHE'LO HAYAH B'OLAMO"
QUESTION: Abaye states that Rebbi Chiya's Halachah does not apply to the Ervah of "Eshes Achiv she'Lo Hayah b'Olamo" because "he does not discuss cases subject to dispute" and the case of "Eshes Achiv she'Lo Hayah b'Olamo" is subject to dispute. Rebbi Chiya's Halachah will apply to such a case only according to Rebbi Shimon, and not according to the Rabanan.
RASHI explains that when Abaye says that "he does not discuss cases subject to dispute," he refers to Rebbi Chiya, and not to the Mishnah, and he means that Rebbi Chiya does not refer to cases subject to dispute.
Why does Rashi explain that Abaye means that Rebbi Chiya does not refer to cases subject to dispute? When Abaye earlier says that "he refers only to cases of Nisu'in," Rashi explains that "he" refers to the Mishnah and not to Rebbi Chiya! Why does Rashi here explain that Abaye's statement (that "he does not discuss cases subject to dispute") refers to Rebbi Chiya?
Moreover, the Gemara earlier (9b) attempts to support the assertion that the Mishnah should have included in its list the case of "Imo Anusas Aviv" (a case which is subject to dispute), and it cites Rebbi Chiya's Halachah as proof that the Mishnah discusses cases of dispute. According to Rashi, however, Rebbi Chiya does not discuss cases subject to dispute!
ANSWER: In the Gemara here, Abaye says that although the Mishnah discusses cases of Nisu'in, Rebbi Chiya applies his Halachah even to the Mishnah's case of "Bito." Although his Halachah does not apply to "Bito" born through Nisu'in (the case the Mishnah discusses) but only to "Bito" born from Onsin (which the Mishnah does not discuss), nevertheless Rebbi Chiya mentions that his Halachah applies to all of the cases in the Mishnah because there is a type of "Bito" to which his Halachah does apply ("Bito" through Onsin).
If Rebbi Chiya discusses any possible case of each Ervah mentioned in the Mishnah, then even if the Mishnah does discuss cases of Machlokes, Rebbi Chiya still can apply his Halachah to "Eshes Achiv she'Lo Hayah b'Olamo," even though it is not the same case as that mentioned in the Mishnah (i.e. it is Rebbi Shimon's case, not the Rabanan's). Accordingly, what difference does it make whether or not the Mishnah discusses cases of Machlokes?
It must be that Abaye means that Rebbi Chiya teaches only the cases which are relevant l'Halachah, even those not mentioned in the Mishnah (such as "Bito" through Onsin); he does not discuss cases which follow only one opinion which the Halachah might not necessarily follow (such as Rebbi Shimon's opinion of "Eshes Achiv she'Lo Hayah b'Olamo").
The Gemara earlier (9b) does not contradict this interpretation. On the contrary, the Gemara there is easier to understand if it means, as Rashi explains here, that "he does not discuss cases subject to dispute" refers to Rebbi Chiya and not to the Mishnah (which does discuss cases subject to dispute). This is because the Gemara there says that the Sugya here proves that the Mishnah must be discussing cases subject to dispute. The Gemara there concludes that Rebbi (who says that the Mishnah is not discussing cases subject to dispute) disagrees with Rebbi Chiya. How, though, does the Gemara there infer from the Sugya here that the Mishnah discusses cases of Machlokes? It is only Rav Safra who says that! Rav Yehudah and Abaye, in contrast, seem to conclude otherwise, that the Mishnah does not refer to cases of Machlokes!
According to Rashi's explanation, the answer is clear. Rav Safra and Abaye argue only whether Rebbi Chiya discusses cases of Machlokes. They do not argue whether the Mishnah discusses such cases. Abaye says that Rebbi Chiya does discuss the case of "Eshes Achiv she'Lo Hayah b'Olamo" even though his Halachah applies to that case only according to Rebbi Shimon, and even though Abaye maintains that Rebbi Chiya only mentions the exact cases that are mentioned in the Mishnah. It must be that the Mishnah does include cases of Machlokes, and Rav Safra and Abaye do not argue about that point. The Gemara there is justified, therefore, when it cites proof against Rebbi that the Mishnah discusses cases of Machlokes.