1) WHEN THE PROHIBITION OF "BITUL MITZVAS YEVAMIM" CAUSES A "CHALITZAH PESULAH"
QUESTION: Shmuel rules that if Leah and her Tzarah were married to one man who died childless, and Rachel and her Tzarah were married to the man's brother who also died childless, the third brother to whom they fall to Yibum must do Chalitzah. If he chooses to do Chalitzah with Leah and Rachel, he also must do a second Chalitzah with their Tzaros, because his Chalitzah with Leah and Rachel was a Chalitzah Pesulah. He may not do Yibum with Leah, the wife of the first brother who died, because she is "Achos Zekukaso" (according to the opinion that maintains "Yesh Zikah"), and he may not do Yibum with Rachel (after he has done Chalitzah with Leah), the wife of the second brother who died, because she is "Achos Chalutzaso."
The Gemara says that according to the opinion that maintains "Ein Zikah," the Tzarah of Leah does not need Chalitzah because there was no Isur of "Achos Zekukaso." RASHI (DH Ela Tzarah d'Leah, and DH Yigmor) explains that the reason why Leah's Tzarah does not need Chalitzah is because the brother could have done Yibum with Leah if he wanted (since she was not prohibited as "Achos Zekukaso" according to the opinion that maintains "Ein Zikah"), and therefore Leah's Chalitzah was not a Chalitzah Pesulah.
Why could he have done Yibum if he wanted? Another prohibition was in effect that should have prohibited him from doing Yibum -- "Isur Bitul Mitzvas Yevamim," the prohibition against forfeiting the Mitzvah of Yibum. (By doing Yibum with Leah, he would forfeit the opportunity to do the Mitzvah of Yibum with Rachel.) Consequently, the Chalitzah he does with Leah still should be a Chalitzah Pesulah.
(a) TOSFOS (DH Ela Tzarah) explains that although it is true that the brother is not permitted to do Yibum because of the Isur of "Bitul Mitzvas Yevamim," nevertheless that Isur is not strong enough to render the Chalitzah a Chalitzah Pesulah. The only prohibitions which cause a Chalitzah to become a Chalitzah Pesulah are "Achos Zekukaso" and "Achos Chalutzaso," since they weaken the bond of Zikah (Tosfos 26b, DH v'Chalitzah).
Tosfos may follow his own opinion as expressed elsewhere. Tosfos (27b, DH Aval Hacha) writes that if a woman was prohibited from doing Yibum because of the Isur of "Bitul Mitzvas Yevamim," she is not considered to have been "Ne'esrah Sha'ah Achas" (forbidden for one moment) such that she remains forbidden forever. Only if the Isur weakens the bond between a Yavam and Yevamah does it cause a permanent Isur of "Eshes Ach" to take effect because of the rule of "Ne'esrah." The Isur of "Bitul Mitzvas Yevamim" does not weaken the bond.
(b) RASHI, however, writes that in practice the surviving brother is permitted to do Yibum with Leah. He does not explain, as Tosfos does, that there is an Isur but that Isur does not render her Chalitzah a Chalitzah Pesulah (see RASHBA, and footnote 62 of Rav Aharon Yaffen zt'l to the Ritva). Rashi maintains that any Isur renders a Chalitzah a Chalitzah Pesulah, as Tosfos (26b, DH v'Chalitzah) quotes in the name of Rashi. (Similarly, Rashi (24a) maintains that the Isur of "Bitul Mitzvas Yevamim" causes an Isur of "Ne'esrah" to take effect.)
Why, though, is there no Isur of "Bitul Mitzvas Yevamim" in this case according to Rashi? RABEINU AVRAHAM MIN HA'HAR implies that according to Rashi, Shmuel follows the opinion (18a) that maintains that there is no such Isur of "Bitul Mitzvas Yevamim."
(c) The TOSFOS HA'ROSH presents another approach to the words of Rashi. He writes that the Isur of "Bitul Mitzvas Yevamim" does not apply in this case. By doing Yibum with Leah, the surviving brother does not avert the Mitzvah of Yibum with the wives of Rachel's husband since he still can do the Mitzvah of Chalitzah with Rachel's Tzarah.
While it is true that once he does Yibum with Leah, Rachel will be forbidden to him because of "Achos Ishto" and obviously he will not be able to do Yibum with her, nevertheless Rachel's Tzarah is not exempt from Chalitzah like any other Tzarah of an Ervah. This is because Rachel became an Ervah after she fell to Yibum. Her Tzarah would have a status of "Tzaras Ervah" only if Rachel would be an Ervah at the time she falls to Yibum. (The brother still may not do Yibum with the Tzarah of Rachel because she is "Tzaras Kerovas Yevimto," who is no different from a "Tzaras Kerovas Chalutzaso.")
The Tosfos ha'Rosh questions this explanation. There still should be an Isur of "Bitul Mitzvas Yevamim" because perhaps the Tzarah of Rachel will die, and her death will cause a forfeiture of the Mitzvah of Yibum with the wives of the second deceased brother (since he cannot do Yibum with Rachel because he already did Yibum with her sister, Leah, and Rachel's Tzarah is dead). The Chachamim prohibited doing Yibum even in such a case, in which there exists only a possibility that the brother will forfeit a Mitzvah of Yibum.
The answer to this question may be that Shmuel refers to the case of the Mishnah in which there are four brothers, and thus there are two brothers who remain and who can do Yibum. When one brother does Yibum with Leah, he does not preclude the Mitzvah of Yibum with Rachel, because even if Rachel's Tzarah dies the other brother can do Yibum with Rachel. The Gemara earlier (26b) teaches that when the forfeiture of the Mitzvah of Yibum will occur only if two people die, there is no concern for that possibility and there is no Isur of "Bitul Mitzvas Yevamim." (Alternatively, the Isur of "Bitul Mitzvas Yevamim" applies only when the Mitzvah will be forfeited if the brother dies; the Isur does not apply when the Mitzvah will be forfeited if the Yevamah dies.) (M. Kornfeld)
2) AN ENDURING PROHIBITION
QUESTION: Rav and Rebbi Yochanan disagree about whether a Yevamah remains forbidden when she was forbidden to the brother at the time that she fell to Yibum but then became permitted. Rav maintains that even when a woman was forbidden and then became permitted, she is allowed to do Yibum. Rebbi Yochanan maintains that she may do Yibum only when she was permitted to the brother at the time she fell to Yibum. If she was initially permitted, became forbidden, and then became permitted again, she may do Yibum.
Rebbi Yosi b'Rebbi Chanina challenges Rebbi Yochanan's view from the Mishnah (26a). The Mishnah states that in a case of four brothers, two of whom were married to two sisters and then died childless, the remaining brothers must do Chalitzah with the two widows of their brothers. According to Rebbi Yochanan, the Mishnah should say that one brother should do Chalitzah with the second sister who fell to Yibum, and the other brother may then do Yibum with the first sister. In such a case, at the moment the first sister fell to Yibum she was permitted, and she became forbidden only later when her sister fell to Yibum. When one brother does Chalitzah with the second sister, the first sister becomes permitted again and thus should do Yibum!
Why does the Gemara not ask the same question on the opinion of Rav? Rav says that even when she was forbidden initially and then became permitted, she remains permitted, and thus the Mishnah certainly is problematic according to Rav.
(a) RASHI (DH Mesah Rishonah) explains that the Gemara's question indeed applies to the opinion of Rav as well. Perhaps the Gemara does not address its question to Rav because of the principle, "Rav Tana Hu u'Palig," Rav has the status of a Tana and may disagree with a Mishnah (as the ME'IRI writes here).
(b) TOSFOS (DH Eisivei) writes that Rav maintains that a Chalitzah Pesulah must be done by all of the brothers. Accordingly, the first sister who fell to Yibum must do Chalitzah with both remaining brothers because she becomes an "Achos Zekukaso" (and thus cannot do Yibum with any of the brothers) once the other sister falls to Yibum. When both brothers do Chalitzah with the first sister, they no longer may do Yibum with the second sister, who becomes forbidden to them because of "Achos Chalutzaso." Hence, Rav would agree with the Halachah of the Mishnah.
In contrast, Rebbi Yochanan maintains that a Chalitzah Pesulah does not need to be done by all of the brothers. Since it suffices for one brother to do such a Chalitzah, the second brother should be able to do Yibum with the other sister, as she is not "Achos Chalutzaso" (because it was not this brother, but the other brother, who did Chalitzah with her sister). Therefore, the Gemara asks its question only on the opinion of Rebbi Yochanan.
(c) According to RABEINU CHANANEL (cited by Tosfos), Rav does not apply the Isur of "Ne'esrah" because he maintains "Ein Zikah," and thus the first sister did not become forbidden when the second sister fell to Yibum. (The Isur of "Bitul Mitzvas Yevamim" does not cause a Yevamah to become forbidden forever; see previous Insight.) Accordingly, the Mishnah presents no problem to Rav, because Rav maintains that the Mishnah follows the view of the Tana'im who maintain "Yesh Zikah," and that is why the Mishnah applies the Isur of "Ne'esrah" (and the Isur of "Achos Zekukaso" remains).
According to Rebbi Yochanan, however, who maintains "Yesh Zikah" (see 28a), the Mishnah poses a problem. Rebbi Yochanan asserts that even if "Yesh Zikah," in a case where a woman was first permitted, became forbidden (because of "Achos Zekukaso") and then became permitted again, she is permitted and the Isur does not endure.
(d) The other Rishonim explain that the Gemara later (28a) provides the solution to this question. The Gemara there explains that the proponents of the Isur of "Bitul Mitzvas Yevamim" and the opinion that "Ein Zikah" will prohibit Chalitzah with the first Yevamah and Yibum with the second, lest the brothers mistakenly reverse the order and thereby cause a forfeiture of the Mitzvah of Yibum. The Gemara proves that Rebbi Yochanan does not agree with that opinion. Rav, however, may follow those who maintain that there is an Isur of "Bitul Mitzvas Yevamim." (See Tosfos here in the name of the RI, and see Tosfos later, 28a, DH Hach.)