ARAYOS THAT UPROOT YIBUM AND CHALITZAH [line 1]
(Mishnah): (Normally, if Reuven died without children, one of his brothers must do Yibum (Levirate marriage) or Chalitzah with one of the widows). If a widow (Esther) is forbidden to a brother (Shimon) by one of the 15 Arayos (forbidden incestuous relationships) listed below, she and all her Tzaros (co-wives) are exempt from (and cannot do) Yibum or Chalitzah with him. (If there is no brother with whom they can do Yibum or Chalitzah, they are permitted to marry strangers.)
If Levi (another brother) did Yibum with Miryam (a Tzarah of Esther) and then he died (childless), all Levi's widows are exempt from Yibum or Chalitzah with Shimon. (Miryam's Tzaros are Tzaros of the Tzarah of Esther.)
The same applies if another brother then did Yibum with a widow of Levi and then died childless, ad infinitum.
The 15 Arayos are:
The daughter of his daughter;
The daughter of his son;
The daughter of Shimon's wife;
The daughter of the son of his wife;
The daughter of the daughter of his wife;
The mother of his mother-in-law
The mother of his father-in-law.
His maternal sister.
His mother's sister;
His wife's sister;
The wife of his maternal brother;
Eshes Achiv she'Lo Hayah b'Olamo (the widow of a brother of Shimon who died before Shimon was born).
If any of these died, did Mi'un (annulled a mid'Rabanan marriage), was divorced, or was found to be an Ailonis (a girl who does not develop like a normal female), the Tzaros are permitted (i.e. obligated to do Yibum or Chalitzah with Shimon or another brother).
Mi'un and being found to be an Ailonis do not apply to the mother and grandmothers of Shimon's wife. (Mi'un must be before adulthood, and a minor cannot give birth. An Ailonis is sterile.)
EXEMPTING THE TZAROS [line 7]
(Continuation of Mishnah): We illustrate the case of Tzaros of a Tzarah. If Shimon's daughter (or any of the other 14 Arayos) Esther was married to Reuven, and Reuven died, just like Esther is exempt (from Yibum with Shimon), so is her Tzarah (Miryam).
If Miryam married (i.e. did Yibum) with Levi, the husband of Tamar, and Levi died, just like Miryam is exempt from Yibum with Shimon, so is Tamar.
This applies even 100 times (a widow does Yibum, and the Yavam dies, his other wives are exempt from Yibum with Shimon).
Question: The Mishnah said that if she died, the Tzaros are permitted. What is the case?
Answer: Shimon's daughter (or another Ervah) Esther was married to Reuven, who had another wife Miryam. If Shimon's daughter died or was divorced, and then Reuven died, Reuven's other wife may do Yibum with Shimon.
If Shimon's daughter could have done Mi'un but didn't, the Tzarah may do Chalitzah (with Shimon) but not Yibum (with him).
ORDER OF THE MISHNAH [line 19]
(Gemara) Question: (Later in our Sugya) we learn all Arayos in the Mishnah from Achos Ishto (his wife's sister). That case should have been taught first!
Suggestion: Perhaps the Mishnah lists the most severe cases first.
We would have to say that the Tana holds like R. Shimon, who says that Sereifah (death by burning) is harsher than stoning.
Objection #1: If so, the Tana should have taught Chamoso (his mother-in-law) first, for the Torah writes Sereifah regarding Chamoso, and from it we learn Sereifah for other Arayos!
Objection #2: If so, his daughter-in-law should follow Chamoso, since (R. Shimon holds that) stoning is the harshest punishment next to burning!
Answer #1: Bito (his daughter) was taught first, because it is not explicit in the Torah. The Tana needed to expound it, therefore he prefers it.