QUESTION: The Mishnah states that if a man (Shimon) has a brother (Reuven)
who is married to a woman (Rachel) who is Asur b'Kares to Shimon as an Ervah
(for example, Shimon's daughter), if Reuven dies without children, then
Shimon may *not* do Yibum or Chalitzah with Rachel. Similarly, he may not do
Yibum or Chalitzah with any of the other wives of Reuven, since they are the
Tzaros of a woman who is an Ervah to Shimon.
The Mishnah lists fifteen such cases of Reuven being married to a woman who
is an Ervah to Shimon. RASHI explains, in each of these cases, how it is
possible for an Ervah to Shimon not to be an Ervah to his brother Reuven and
to be married to him. When discussing the case of "Chamoso," Shimon's mother-
in-law (the mother of Shimon's wife, who is an Ervah to Shimon), Rashi
explains that Reuven had married Shimon's mother-in-law after her first
husband died, and that is how she came to be married to Reuven (her
daughter's husband's brother).
Why does Rashi not give a much simpler scenario for how Reuven could be
married to Shimon's mother-in-law: Shimon married Reuven's daughter, his
niece! Shimon's mother-in-law is Reuven's wife, and when Reuven dies, she
cannot do Yibum with Shimon because she is an Ervah to him! (MAHARAM)
ANSWER: The RIVASH (#374), REBBI AKIVA EIGER and others explain that Rashi
here is consistent with his opinion elsewhere. Later in Yevamos (94b), the
Gemara cites an argument between Rebbi Yishmael and Rebbi Akiva regarding the
Isur of marrying one's mother-in-law (Chamoso). According to Rava's
understanding of the argument, Rebbi Akiva rules that if one marries his
mother-in-law after the death of one's wife, the Isur is no longer punishable
with Sereifah, while Rebbi Yishmael holds that the Isur is still punishable
with Sereifah. Tosfos there asserts that the argument involves only the
punishment of Sereifah, and everyone agrees that there is still a Lo Ta'aseh
that prohibits marrying one's mother-in-law even after the death of his wife.
However, RASHI in Sanhedrin (76b) writes that according to Rebbi Akiva, there
is *no* Isur Kares or Lo Ta'aseh at all for marrying one's mother-in-law
after the death of one's wife. One's mother-in-law is only "Asur b'Arur" --
the Torah places a curse on one who marries his mother-in-law, but it is not
included in a distinct negative prohibition. Hence, since she is not an
actual Ervah to him, in a case where she was married to his brother and she
falls to him in Yibum, she obviously is not exempt from Yibum, nor can she
exempt her Tzaros from Yibum.
In the case in the Mishnah, Reuven died without children. If Rachel, Reuven's
wife, was Shimon's mother-in-law and their daughter was Shimon's wife, then
the only possible way Rachel would fall to Shimon in Yibum is if Reuven's
daughter -- Shimon's wife -- had died! At that point, after the death of
Shimon's wife, his mother-in-law is no longer forbidden to him as an Ervah,
but only "b'Arur," and therefore she must do Yibum or Chalitzah with Shimon
before remarrying, and she will not exempt the Tzaros!
That is why Rashi had to explain that the Mishnah's case of "Chamoso" is when
Reuven, the brother who died, had married the mother-in-law of Shimon, the
surviving brother, after her husband (Shimon's wife's father) died.