Answer #1: Rebbi disagrees with these rules.

Answer #2 (Rav Ada): Really, Rebbi agrees with these rules. He derided Levi because both rules cannot apply (simultaneously) to Imo Anusas Aviv.

If Yakov raped two sisters (and Shimon and Yehudah resulted, and the sisters married other sons of Yakov), we can fulfill two sisters falling to Yibum, but we cannot fulfill that each is permitted to a different brother (she is the mother of one Yavam, and Dodaso of the other (his mother's sister);

If he raped two unrelated women, we fulfill that each is permitted to a different brother (the one who is not her son), but they are not sisters.

Answer #3 (Rav Ashi): Rebbi disagrees with these rules. The Mishnah discusses cases of dispute;

Rebbi derided Levi for not deducing from the coming Mishnah (#2) that the Tana is R. Yehudah (who forbids a man to a woman raped by his father. We assume that the same Tana taught the entire Perek.)

(Mishnah #2): Six Arayos are more severe than these (of the first Mishnah). They can marry only others (but not any of the brothers), therefore their Tzaros are permitted: Imo (the Yavam's mother), his father's wife, his father's sister...

Question: What is the case of Imo?

We cannot say that she was married to his father, for that is listed second!

Answer: Rather, she was raped by his father, and the Mishnah says that she can marry others, but not the brothers!

This is like R. Yehudah, who forbids Anusas Aviv. This is why the first Mishnah omitted this case!

Question (Ravina): Also according to R. Yehudah this may be taught (in the first Mishnah), if he transgressed and married her!

Answer (Rav Ashi): The Tana does not list cases that come through forbidden marriages.

Question (Rav Ashi): The case can arise through permitted marriages!

If Yakov raped the wife of his son Reuven, and Shimon was born, and Reuven died without children, his widow falls to Yibum to her son.

Since she is forbidden, her Tzarah is also forbidden.

Answer (Rav Kahana): The Tana discusses only cases when the brothers are not the product of Isur.

Still, in spite of Rebbi's objection, Levi included this is in his (version of the) Mishnah:

Sometimes Shimon's mother exempts her Tzarah, sometimes not. If she was married to the father, she does not exempt the Tzarah. If she was raped by the father and married a brother, she exempts the Tzarah.

Even though we learned that 15 cases exempt the Tzarah, we find this 16th case.

Question (Reish Lakish): Levi holds that b'Di'eved cases are listed. If so, the Mishnah should include one who did Chalitzah to his Yevamah, then he was Mekadesh her and died without children;

Since she may not do Yibum, also her Tzarah is exempt!

Answer (R. Yochanan): This case is omitted, since there cannot be the Tzarah of a Tzarah (since the Tzarah cannot do Yibum with any brother).

Question: Why didn't R. Yochanan answer that a Yevamah after Chalitzah is Chayavei Lavin, so she does Yibum or Chalitzah!

Answer: R. Yochanan answered according to Reish Lakish's opinion:

I (R. Yochanan) hold that a Yevamah after Chalitzah is Chayavei Lavin, so she does Yibum or Chalitzah. You (Reish Lakish) hold that this is Chayavei Kerisos. You can say that it was omitted because the case of Tzaras Tzarah does not arise.

(Reish Lakish): If one was Mekadesh his Yevamah after Chalitzah, he is not forbidden to her with Kares, but the brothers are. Both he and the brothers are forbidden to a Tzarah with Kares;

(R. Yochanan): There is not Kares for him or the brothers, not for her and not for the Tzaros.

Question: What is Reish Lakish's reason?

Answer: "Who will not build the house of his brother" - once he decided not to build (did Chalitzah), he may not build.

This applies (only) to the Choletz (the Yavam who did Chalitzah). His brothers are forbidden like they used to be (there is Kares without a Mitzvah of Yibum to permit it).

This applies only to the Choletzes (who did Chalitzah). Her Tzaros are forbidden like they used to be.

R. Yochanan disagrees, for we never find that initially any brother may do Chalitzah to any widow (and then only a Lav would forbid her), and now (that he did not do Chalitzah to her), he is forbidden to her with Kares.

Rather, the Choletz is a Shali'ach on behalf of all the brothers, and the Choletzes is a Shali'ach on behalf of her Tzaros.

Question (R. Yochanan - Beraisa): If one did Chalitzah to his Yevamah, then was Mekadesh her and he died, she must do Chalitzah with the brothers.

This is like me. Since only a Lav forbids her to the brothers, Chalitzah is needed.

However, you say that she is forbidden to the brothers by Kares. If so, why is Chalitzah needed?

Counter-question (Seifa): If one of the brothers was Mekadesh her, she has no claim against him (i.e. the Kidushin did not take effect).

If it is only Chayavei Lavin, why is this?

Answer #1 (Rav Sheshes): The Seifa is R. Akiva, who says that Ein Kidushin Tofsin b'Chayavei Lavin (if a man is Mekadesh a woman forbidden to him by a Lav, it does not take effect).

Objection: If so, it should say that according to R. Akiva, she has no claim against him! (The Reisha cannot be R. Akiva, for if so Kidushin would not take effect even if the Choletz was Mekadesh her.)

This is left difficult.