QUESTION: The Mishnah discusses fifteen cases in which a man ("Shimon") has a brother ("Reuven") whose wife ("Rachel") is forbidden to him as an Ervah (such as his daughter). The Mishnah teaches that if Reuven dies without children, Shimon may not perform Yibum or Chalitzah with Rachel. Similarly, he may not perform Yibum or Chalitzah with any other wife of Reuven, because she is the "Tzarah" ("co-wife") of a woman who is an Ervah to Shimon.
The Mishnah lists fifteen different cases in which Reuven is married to a woman who is forbidden as an Ervah to Shimon. In each case, RASHI explains how it is possible for the woman to be an Ervah to Shimon but not to his brother, Reuven. In the case of "Chamoso," Shimon's mother-in-law (Shimon's wife's mother, who is an Ervah to Shimon), Rashi explains that Reuven married Shimon's mother-in-law after her first husband died. That is how she came to be married to Reuven (her daughter's husband's brother).
There is a far more obvious explanation for how Reuven could be married to the mother-in-law of his brother, Shimon. Shimon simply married his niece, Reuven's daughter, in which case Shimon's mother-in-law is Reuven's wife. Hence, when Reuven dies, Shimon may not perform Yibum with her because she is an Ervah to him. Why does Rashi not give this simple explanation for how Reuven can be married to his brother's mother-in-law? (MAHARAM)
ANSWER: The RIVASH (#374), REBBI AKIVA EIGER, and others explain that Rashi here follows his own opinion as expressed elsewhere. The Gemara later in Yevamos (94b) records a dispute between Rebbi Yishmael and Rebbi Akiva with regard to the prohibition against marrying one's mother-in-law (Chamoso). According to Rava's understanding of the dispute, Rebbi Akiva rules that the punishment for marrying one's mother-in-law (Sereifah) does not apply to one who marries her after the death of his wife. Rebbi Yishmael argues and rules that the punishment still applies in such a case. TOSFOS there asserts that their dispute involves only whether or not the punishment of Sereifah applies; everyone agrees that a Lo Ta'aseh still prohibits one from marrying his wife's mother even after the death of his wife.
RASHI in Sanhedrin (76b), however, writes that according to Rebbi Akiva no Isur Kares or Lo Ta'aseh prohibits one from marrying his wife's mother after the death of his wife. His wife's mother is only "Asur b'Arur" to him -- the Torah places a curse on a man who marries his mother-in-law; no explicit Torah prohibition forbids it. Accordingly, since his mother-in-law is not an actual Ervah to him, in a case in which she was married to his brother who died with no children, she is not exempt from Yibum and she does not exempt her Tzarah from Yibum.
In all of the cases of the Mishnah, Reuven died with no children. In the case of "Chamoso," Reuven's wife, Rachel, is the mother of Shimon's wife, which means that Shimon is married to the daughter of Reuven. In such a case, however, the only possible way for Rachel to fall to Shimon for Yibum is if Reuven's daughter -- Shimon's wife -- is dead (because if she is alive, Reuven has a child and there is no obligation for his wife to perform Yibum). However, if Shimon's wife is dead, his mother-in-law is no longer forbidden to him as an Ervah but only "b'Arur," and thus she must perform Yibum or Chalitzah with Shimon before she may remarry (and she does not exempt her Tzarah from Yibum)!
For this reason, Rashi explains that the Mishnah's case of "Chamoso" refers to when Reuven, the brother who died, had married the mother-in-law of Shimon, the surviving brother, after her husband (the father of Shimon's wife) died.