Question (Levi bar Susai, to Rebbi): We should teach 16 women [in our Mishnah, i.e. also] his mother, whom his father raped (or enticed, but never married her). She can marry his paternal brothers [from a different mother, so Tzaras Tzarah applies]!

Rebbi (to Levi): It is evident that that man (you) has no brain in his skull.

Question (Levi): Why [do you utterly reject this]?

Answer #1 (Rebbi): It is due to R. Yehudah. R. Yehudah forbids a man to a woman whom his father raped or enticed (so she cannot marry his paternal brothers. Our Mishnah is like R. Yehudah. Alternatively, our Tana taught only what all agree to. He does not discuss arguments.)

Objection: Doesn't R. Yehudah agree that if he was Mekadesh her, Kidushin takes effect on her?! (She is only Chayavei Lavim, without Kares!)

Answer #2: Rather, it is because the next Mishnah taught six Arayos are more stringent than these [15]. Because they can marry [only] others (but not brothers of the Yavam), their 'Tzaros' are permitted (they are not really Tzaros, for the Ervah is not truly married);

And R. Chiyah taught that if they can marry his brothers without an Aveirah, their Tzaros are exempt from Chalitzah or Yibum, and of all [six], the only one [who can do so] is his mother, whom his father raped, and she married his paternal brother.

Question: Why didn't [Rebbi] teach it [in our Mishnah]? If it is because R. Chiyah taught it in a Beraisa, he did not teach it, [this is unreasonable]. All the more so he should teach it!

Answer: Rather, it is because we taught that when [a Yevamah's] sister is her Yevamah (i.e. both fell to Yibum from different brothers, and there are two surviving brothers, and each sister is forbidden to a different brother and permitted to the other), she does Chalitzah or Yibum, and you can find regarding all of [the 15 women] that her sister is her Yevamah, she does Chalitzah or Yibum, except for his mother, whom his father raped, and she married his paternal brother. (This case cannot work, for one is forbidden to his mother's sister.)