(Mishnah): If two of four brothers were married to sisters, and these two died, the widows do Chalitzah, and not Yibum. If they did Yibum, they must be divorced;

R. Eliezer says, Beis Shamai allow them to stay married. Beis Hillel say that they must be divorced.

(Gemara) Question: If two of four brothers were married to strangers, do [the widows] not do Yibum? (Why is it different if they were married to sisters?)

Answer: Here is different, for there is an Isur of sisters (Achos Zekukaso).

Question: If so, they should not need Chalitzah!

Answer (R. Masniyah): The Isur is through Zikah. The Isur of sisters (Achos Zekukaso) is not so clear (strong enough to exempt without Chalitzah).

(Mishnah): A Shomeres Yavam, if [a Yavam's] brother (Levi) was Mekadesh her sister - in the name of R. Yehudah ben Beseira, they said that we tell [Levi] to wait until [his brother] does an action (Yibum or Chalitzah);

If [one of] the brothers did Chalitzah or married her (did Yibum, Levi) may marry [his Arusah]. If the Yevamah died, he may marry [his Arusah]. If the Yavam died, he divorces his wife with a Get and does Chalitzah to his brother's wife.

Limitation: It says only that if his Yevamah died, he is permitted to his wife (Arusah), but if his wife died, he is forbidden to his Yevamah.

(R. Yochanan): These are R. Lazar's words, but Chachamim say that if his Yevamah died, he is permitted to his wife. If his wife died, he is permitted to his Yevamah.

Chachamim say that any matter [Isur] that comes due to a cause (the Yevamah was forbidden to Levi due to her sister's Kidushin), when the cause is Batel, the Isur is Batel.

R. Lazar holds that even when the cause is Batel, the Isur is in its place.

Question (R. Yosi bei R. Chaninah, to R. Yochanan): Here you say that if his Yevamah died, he is permitted to his wife. If his wife died, he is permitted to his Yevamah, and here (in our Mishnah) you say so?! (After a brother does Chalitzah to one, the other is no longer Achos Zekukaso. The cause is Batel, so the Isur to do Yibum with the other should be Batel!)

R. Yochanan: I do not know the reason for [Chachamim in that Mishnah of] sisters. (Since the Yevamah was forbidden, why is she permitted afterwards?)

Support (R. Ze'orah citing R. Chiyah citing R. Yochanan): I do not know the reason for [that Mishnah of] sisters.

(R. Ba citing R. Chiyah citing R. Yochanan): I do not know the reason for [our Mishnah of] sisters who are Yevamos. (The one who fell to Yibum first was initially permitted. She should return to be permitted!)

Answer #1 (R. Heila citing R. Yosah citing R. Yochanan): The Isur of sisters who are Yevamos is unlike the Isur of sisters who are not Yevamos.

Answer #2 (R. Yirmeyah) The case is, the house fell on both [brothers, and they died] at once. (Neither Yevamah was initially permitted.)

Objection (R. Yosah): If the house fell on both of them at once, would R. Yochanan say 'I do not know the reason for sisters who are Yevamos'?! Rather, they died one after the other.

Suggestion: They wanted to say that R. Yochanan asked, if the second died, why is he not permitted to the first? However, if the first died, [he understands why] he is forbidden to the second (she was forbidden when she first fell).

Rebuttal (R. Yehudah): Both of these are the same. He asked about both of these. Zikah is nothing (i.e. it is not strong enough to forbid even after one of the Yevamos died. Only if when she fell she was forbidden due to a strong Isur, she is forbidden forever);

Source: If there are three brothers - two (Reuven and Levi) are paternal, but not maternal, brothers, and two (Levi and David) are maternal, but not paternal, brothers, and Reuven died, and before Levi could do Chalitzah or Yibum, he died, and she 'fell' to his maternal brother (David), is she not permitted to him?! (Chalitzah and Yibum do not apply, for there are no living paternal brothers.)

If you will say that Zikah is something (an Isur remains even after Zikah is Batel), he should be forbidden to her, due to his maternal brother (when she was Zekukah to Levi, it is as if she was married to him! Rather, Zikah is nothing after death.)

Question: [If two sisters fell to Yibum from two brothers,] what is the law if [one of them] has a Tzarah? (Zikah suffices to forbid the sisters. Does it forbid the Tzarah?)

Answer (R. Heila): A Kal v'Chomer teaches [that it forbids]. Achos Chalutzaso, which is mid'Rabanan, Chachamim gave to her an [Isur on her] Tzarah. Here that there is an Isur of sisters (Achos Zekukaso), all the more so! (This connotes that Zikah is mid'Oraisa; however, according to PNEI MOSHE there is no proof.)

His daughter - obviously, she has a Tzarah. (OHR SOMAYACH Kuntres Zikah 15 - if one of the sisters was Ervah to a Yavam, it is as if she did not fall, so he may do Yibum with her sister. Similarly, if a Tzarah of a sister was the Yavam's daughter (or another Ervah), that sister is like Tzaras Ervah, and it is as if she did not fall, so the other sister may do Yibum.)

(R. Bun bar Chiyah citing R. Avina): It was taught there that if the Tzaros [of the sisters] did Chalitzah, the sisters are exempted. If the sisters did Chalitzah, the Tzaros are not exempted. (Since the sisters were forbidden to do Yibum, their Chalitzah is Pasul.)

Question: Just like you say that if the Tzaros did Chalitzah, the sisters are exempted (even though the Tzaros may not do Yibum, for since the sisters are forbidden, they are like Ervah, so the Tzaros are like Tzaras Ervah), you should similarly say that if the sister did Chalitzah, the Tzaros are exempted!

Answer #1: This is like R. Yochanan ben Nuri;

R. Yochanan ben Nuri: Let us enact that the Tzaros do Chalitzah, and not Yibum!

Objection: Was this enacted?! Was it not taught that they were unable to enact before the opportunity was lost?!

Answer #2: It is like Beis Shamai. Beis Shamai permit the Tzaros to the brothers.

Question (R. Pinchas, to R. Yosi): If it is like Beis Shamai, even if the sisters did Chalitzah, the Tzaros should be exempted (since they hold that if the sisters did Yibum, they remain married. It is as if they are proper to do Yibum!)

(Mishnah - R. Lazar): Beis Shamai say, if they did Yibum, they keep [their wives]. Beis Hillel say, they divorce them.

Answer: Two Tana'im argue about Beis Shamai's opinion. One says that if the sisters did Chalitzah, the Tzaros are exempted. The other says that the sisters did Chalitzah, the Tzaros are not exempted.