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A woman is not believed to testify that her Yevamah died or that her sister died.
A man is not believed to testify that his brother died or that his wife died.
Rebbi Akiva maintains that a child born from an Isur Lav is a Mamzer.
Rava says that the testimony of a woman that her Yevamah died is believed. (1)
A Get that divorces a woman but does not allow her to remarry is a "Rei'ach ha'Get," and it disqualifies the woman to a Kohen.
If witnesses erroneously testify that a man's wife died and then he married her sister, he is permitted to his wife. (2)
If witnesses testify that a man's wife died and then he married her sister, and subsequently the witnesses say that she was alive at the time but now she has died, the children born before his wife died are Mamzerim.
The only Ervah who requires a Get is an Eshes Ish who remarried on the basis of the testimony of a single witness. (3)
The sister of one's divorced wife is forbidden.
Kidushin can be conditional, but not Nisu'in. (4)
Rava says that Rebbi Yishmael and Rebbi Akiva disagree about whether a relationship with a mother-in-law after the death of her daughter is punished with Sereifah.
A BIT MORE
1. This is derived from a Kal va'Chomer. If we believe her testimony regarding an Isur Kares (an Eshes Ish is believed regarding the death of her husband), then certainly we believe her regarding an Isur Lav (a Yevamah who testifies that her Yavam died).
2. He is permitted to the relatives of the sister. If his wife subsequently dies, he may marry the sister.
3. Rebbi Akiva says that a man who marries his brother's wife or the sister of his wife also requires a Get.
4. Nisu'in is never conditional, because a person does not make his Bi'ah into a Bi'as Znus.
THE EMOTION OF HATE VERSUS THE EMOTION OF LOVE
The Gemara says that a woman may not be believed regarding her testimony that her Yavam died, because perhaps she hates the Yavam and is testifying falsely to escape his clutches. Why, then, is a woman believed to say that her husband died and that she must do Yibum? Perhaps her testimony is based on her love of the Yavam, and she is testifying that her husband died so that she may marry the Yavam! Tosfos answers that the emotion of hate is much stronger than that of love. For that reason, a woman will testify falsely out of hate for the Yavam, but she will not testify falsely out of love for the Yavam.
TESTIMONY REGARDING YIBUM
The testimony of a single witness is believed when he says that a woman's husband died and she is Zekukah to Yibum, or that her Yavam died and she is free to remarry, or that her husband died followed by her son (and she is free to remarry). Even a servant, a woman, and a Kusi are believed if they mention in the course of a conversation that the Yavam died. A witness who testifies in the name of another witness is also believed to testify that the Yavam died, just as he is believed to testify that the husband died. Some say, however, that a single witness is not believed to testify that the Yavam died. (Shulchan Aruch EH 158:3)
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