Suggestion: Perhaps this is a decree lest one do Chalitzah with the first and Yibum with the second (but if Chalitzah was done with the second, the first may do Yibum)!

Rejection: 'They do not do Yibum' connotes that it is always forbidden.

Suggestion: Perhaps we decree lest the brother who does not do Yibum die, for it is forbidden to Mevatel the Mitzvah of of Yibum!

Rejection: R. Yochanan holds that we are not concerned lest one die.

Suggestion: Perhaps the Mishnah is R. Eliezer, who says that once a Yevamah became forbidden, she remains forbidden forever!

Rejection: Since the Seifa mentions R. Eliezer, he did not teach the Reisha.

Suggestion: Perhaps they fell simultaneously, like R. Yosi ha'Galili, who says that this is possible!

Rejection: We do not have a Stam Mishnah like R. Yosi ha'Galili.

Suggestion: Perhaps we do not know which fell first!

Rejection (Seifa): If they both did Yibum, they must divorce their wives.

Granted, the first to do Yibum must divorce her, for she was forbidden to him.

Why must the second divorce his wife? He can say that his brother did Yibum with the second, and he did Yibum with the first!

Conclusion: This is why R. Yochanan said that he did not know who authored our Mishnah.

(Mishnah): If one sister was Ervah to a Yavam (Reuven), he is forbidden to her, and permitted to marry her sister. The other brother is forbidden to both.

Assumption: Reuven's (e.g) mother-in-law (i.e. the sister who is Ervah to him) fell first.

Question: Reuven can do Yibum with her sister, and his mother-in-law will then be a Yevamah who was permitted (to the other Yavam), forbidden, and permitted again!

Answer (Rav Papa): The case is, the other sister fell first.

(Mishnah - R. Eliezer): Beis Shamai say (if they did Yibum, they may remain married).

(Beraisa - R. Eliezer): Beis Shamai permit them to remain married; Beis Hillel say, they must be divorced;

R. Shimon says, they may remain married;

Aba Sha'ul says, Beis Hillel are more lenient and permit them to remain married, and Beis Shamai say that they must be divorced.

Question: Whose opinion does R. Shimon express?

If he gives Beis Shamai's opinion, R. Shimon is like R. Eliezer!

If he gives Beis Hillel's opinion, he is like Aba Sha'ul!

Answer: He says that Beis Shamai and Beis Hillel did not argue about this.

(Mishnah): If one was (Ervah to Reuven, he is forbidden to her and permitted to her sister. Shimon is forbidden to both).

Question: We learned this in a previous Mishnah!

(Mishnah): A woman who falls to Yibum with her sister (who is Ervah to the Yavam) does Chalitzah or Yibum.

Answer: Both Mishnayos are needed.

In that Mishnah there is only one Yavam. Here, one might have thought that we forbid Yibum, lest Shimon (who is not related to either Yevamah) do Yibum.

In our Mishnah, Shimon must do Chalitzah, so this shows that Achos Zekukaso is forbidden. There, there is no other brother, so one might have thought that we forbid Yibum lest people think that Achos Zekukaso is permitted.

(Mishnah): If one was forbidden due to an Isur Mitzvah (or Isur Kedushah, she does Chalitzah but not Yibum).

Question: We already learned this in another Mishnah!

(Mishnah): If she is forbidden because of Mitzvah or Kedushah, she does Chalitzah, but not Yibum.

Answer: There, Leah fell alone to Yibum. Here, her sister Rachel fell with her;

One might have thought Chachamim consider an Isur Mitzvah to be like Ervah to allow Rachel to do Yibum. Our Mishnah teaches that it does not.

Question: What is the reason?

Answer: Since mid'Oraisa Leah falls to Yibum, Rachel is forbidden due to Achos Zekukaso.

(Mishnah): If one sister was Ervah to one Yavam, and the other was Ervah to the other, each Yavam may do Yibum with the Yevamah who is not Ervah to him.

Question: We already learned this in the Reisha! What difference does it make if only one is Ervah to a Yavam, or if each is Ervah to a different Yavam?!

Answer: Had it taught only about when only one is Ervah, we would have thought that only there Yibum is allowed. Since the other Yevamah must do Chalitzah, this shows that Achos Zekukaso is forbidden (but here we would decree to forbid Yibum, lest people think that Achos Zekukaso is permitted);

Had it taught only about when each is Ervah, we would have thought that only then Yibum is allowed. Since each Yavam is forbidden to a Yevamah, people realize that there is no Achos Zekukaso, due to Ervah. However, when there is only one Ervah, perhaps we decree not to do Yibum.

(Mishnah): This is the case in which the sister (of Ervah), who also falls to Yibum, may do Chalitzah or Yibum.

Question: What does this come to exclude?

Answer: It excludes when each sister is an Isur Mitzvah to a different Yavam.

Question: We already learned about when one is an Isur Mitzvah to a Yavam. Surely this is the same!

Answer: We might have thought that there we do not consider the Isur like Ervah, lest the other Yevamah do Yibum (and she is Achos Zekukaso). Here, we would consider each Isur Mitzvah to be Ervah, and each may do Yibum with the other Yavam;

The Mishnah teaches that this is not so.

(Rav Yehudah citing Rav): For every Ervah (in the Mishnah (2a)), we can find that the Yevamah forbidden to one Yavam due to that Ervah is permitted to the other Yavam; and when they are sisters (falling from different brothers), they do Chalitzah or Yibum.

Rav Yehudah himself applies this only to the Arayos starting with one's mother-in-law.

He does not apply this to the first six Arayos, since (they are branches of the Isur of Bito, and) the case of Bito is only found through rape, but not through marriage. The Tana does not discuss such cases.

Abaye says that even Bito is included, even though it does not arise through marriage.

Abaye excludes Eshes Achiv she'Lo Hayah b'Olamo, since it arises only according to R. Shimon;

The Tana does not discuss cases subject to dispute.

Rav Safra includes even this case. It arises with six brothers, according to R. Shimon.

A way to remember the case is 'Died, was born, did Yibum; died, was born, did Yibum'.

Reuven and Shimon were married to two sisters; Levi and Yehudah were married to two unrelated women. Reuven died, Yisachar was born, Levi did Yibum; Shimon died, Zevulun was born, and Yehudah did Yibum. Levi and Yehudah died without children.

Reuven's widow is forbidden to Yisachar and Shimon's widow is forbidden to Zevulun; they are sisters.

Question: This is true even if Yehudah didn't do Yibum!

Answer: We discuss when Yehudah did Yibum so that each Tzarah will be permitted to a different brother.

Question: This shows that the case applies to a Tzarah. How can it apply to Tzaras Tzarah?

Answer: Later, Gad and Asher (other brothers) did Yibum with the Tzaros.

(Mishnah): If two of three brothers were married to sisters, or to a woman and her daughter or granddaughter; (if the two brothers die) the Yevamos do Chalitzah but not Yibum;

R. Shimon says, there is no need for Chalitzah.

If one of the sisters was forbidden to a Yavam due to Ervah, he is forbidden to her and permitted to her sister;

If one of the sisters was forbidden to a Yavam because of Isur Mitzvah or Kedushah, they do Chalitzah, but not Yibum.

(Gemara - Beraisa): R. Shimon exempts from Yibum and Chalitzah - "Do not marry a woman with her sister to be rivals" - when they are rivals to each other (both fell to Yibum to the same brother), you cannot marry even one of them.

(Mishnah): If one of them was forbidden because of Ervah...

Question: We already learned this (in the first Mishnah of our Perek)!

Answer: It is taught to teach R. Shimon's opinion (that he agrees);

(When neither sister is Ervah to a brother,) R. Shimon forbids sisters to do Yibum. One might have thought that we should decree when one is Ervah due to when neither is.