DOES ONE WITNESS PERMIT A YEVAMAH?
Answer #1 (Rav Sheshes - Mishnah #1): A woman's husband and son went overseas. They told her that her husband died and then her son died. She remarried. Later, they told her the son died first (so she needed to do Yibum or Chalitzah). She must leave her husband. Children she had from him, both the first and last, are Mamzerim.
Question: What is the case?
Answer #1: Two witnesses told her the first time, and two witnesses later contradicted them.
Objection #1: Why should we believe the second witnesses more than the first?
Objection #2: The child is only a Safek Mamzer!
Suggestion: Perhaps the Tana was not so exact.
Rejection (Seifa): The first children are Mamzerim, but later children are not.
This shows that the Tana was exact!
Answer #2: Rather, one witness told her the first time, and later two not contradicted him.
Inference: Had he not been contradicted, he would have been believed!
Rejection (and Answer #3 to Question a:1): We can say like Rav Acha bar Minyomi said (elsewhere), it is a case of Edim Zomemim. (Two witnesses testified, and two others said 'You could not have seen this, for you were with us elsewhere at that time.' We believe the latter witnesses.)
Answer #2 (Rav Mordechai - Mishnah #2): A woman is not believed to say 'My Yavam died' to marry a stranger, nor to say 'My sister died' to marry her sister's husband.
Inference: She is not believed, but one witness would be believed!
Rejection (Rav Ashi - Seifa): A man is not believed to say 'My brother died' so he will do Yibum, nor to say 'My wife died' to marry her sister.
If you make the above inference, you should also infer here that he is not believed, but one witness is!
Objection: Granted, Chachamim were lenient to believe one witness to permit a woman lest she be an Agunah (unable to remarry). This does not apply to marrying one's wife's sister!
Answer: Rather, Mishnah #2 is like R. Akiva. Since he holds that a child of Chayavei Lavin is a Mamzer, we would have thought that a Yevamah is concerned for her children and will check well before marrying, so she should be believed;
The Mishnah teaches that this is not so. She is concerned only for her own ruin, but not for that of her children. (Rashi rejects this text, for according to R. Akiva, also a Yevamah who married a stranger is ruined. Tosfos 92b DH Aval - Mishnah #2 is no Chidush according to Chachamim. Our Sugya permits a Yevamah to do Yibum even after she was Mezanah or married l'Shuk. Since she will not be fined, obviously she is not believed!)
Answer #3 (Rava): A Kal va'Chomer teaches that one witness is believed about a Yevamah:
One witness permits Chayavei Kerisus (Eshes Ish), all the more so he permits Chayavei Lavin!
Objection (a Chacham): A woman's credibility refutes your reasoning!
She is believed for Chayavei Kerisus (if she says 'my husband died'), but she is not believed for Chayavei Lavin (to say that her Yavam died)!
This is because sometimes she hates the Yavam, and she will not check well. The same applies when one witness says that the Yavam died!
(Mishnah - R. Elazar ben Masya): "And a woman divorced from her husband (is forbidden to a Kohen)", but not a woman divorced from a man who is not her husband.
Rav Yehudah: R. Elazar ben Masya should have expounded a pearl. Instead, he expounded shards!
Question: What pearl should he have expounded?
Answer (Beraisa): "And a woman divorced from her husband (is forbidden to a Kohen)", even if she was only divorced from her husband (the Get says 'you are divorced from me but not permitted to others)!
This is 'the scent of a Get' which forbids to Kehunah.
A MAN WHO MISTAKENLY MARRIED HIS WIFE'S SISTER
(Mishnah): Reuven's wife Leah went overseas. They told him that she died; he married her sister Rachel. Later, Leah returned. She may return to him, and he is permitted to Rachel's relatives, and Rachel is permitted to his relatives. If Leah later died, he may marry Rachel;
If they told him that Leah died, he married Rachel, and they told him that really, she died after the Nisu'in to Rachel, the first child is a Mamzer, but the latter is not;
R. Yosi says, anyone who forbids others forbids to himself. Anyone who does not forbid others does not forbid to himself (this will be explained).
(Gemara) Inference: The Mishnah applies even if Leah went abroad with Rachel's husband (and the witness said that both of them died), and when Reuven married Rachel, this forbade Rachel to her husband. Still, this marriage does not forbid Reuven to Leah.
Suggestion: Our Mishnah is unlike R. Akiva. He would forbid Reuven to Leah, since she is the sister of his divorcee!
(Beraisa): The only Ervah that requires a Get (if married mistakenly) is an Eshes Ish who remarried according to Beis Din;
R. Akiva adds Eshes Ach and Achos Ishto.
Since Rachel needs a Get from Reuven, she is like his divorcee, so Reuven is forbidden to her sister Leah!
Rejection: If we explain the Beraisa like Rav Gidal, the Mishnah can be like R. Akiva.
(Rav Gidal): The case of Eshes Ach who needs a Get is when Levi was Mekadesh Sarah, and he went overseas. Levi's brother Yehudah heard that Levi died, and Yehudah married (did Yibum with) Sarah.
People will say that Levi's Kidushin was on condition and Yehudah properly married her.
The case of Achos Ishto is similar. Levi was Mekadesh Sarah, and she went overseas. Levi heard that she died, and married her sister.
People will say that Levi's Kidushin was on condition and he properly married her sister.
Our Mishnah discusses Nisu'in. People will not say there was a condition by Nisu'in (the husband would not want to transgress Bi'as Zenus if the condition is not fulfilled), so R. Akiva agrees that no Get is needed.
A MOTHER-IN-LAW AFTER THE WIFE DIES
Question (Rav Ashi): If the Mishnah is R. Akiva, it should also teach [about marrying] Chamoso (his mother-in-law) after he heard that his wife died. R. Akiva holds that one is not Chayav Misah for Chamoso after his wife dies!
(Beraisa - R. Yishmael): "You will burn him and them (a woman and her mother)" - him and one of them (the one whom he married after marrying the other);
R. Akiva says, (we burn) him and both of them.
We understand according to Abaye, who says that they agree about the Halachah, but argue only about how do derive it. R. Yishmael holds that the verse discusses only one Bi'as Isur. R. Akiva holds that it discusses marrying Chamoso and her mother.
However, Rava says that they argue about Chamoso after one's wife dies (R. Akiva expounds that one is liable only if both women are still alive). R. Akiva should have taught also about Chamoso!
Answer (Rav Kahana): Granted, R. Akiva says that one is not Chayav Misah for her, but the verse did not permit her!
Question: Why doesn't Reuven become forbidden to his wife when he married Achos Ishto, just like a woman who married after she heard that her husband died?
Answer: The cases are different. An Eshes Ish who is Mezanah b'Mezid is forbidden mid'Oraisa to her husband, so Chachamim decreed to forbid her for Shogeg;
Zenus b'Mezid with Achos Ishto does not forbid one to his wife mid'Oraisa, so no decree was made for Shogeg.