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Rebbi Elazar says that a Yevamah who is forbidden to the Yavam even for one moment is forbidden forever.
If two sisters fall to Yibum to two brothers, and one of the sisters is an Ervah to one brother, and the sister that was not the Ervah fell to Yibum first, the brother who is an Ervah to one of the sisters may do Yibum to the other sister, while the other brother is forbidden to both of them. (1)
If two sisters fall to Yibum to two brothers and one of the sisters is an Isur Mitzvah (prohibited mid'Rabanan) to one of the brothers, he may do only Chalitzah and not Yibum. (2)
If two sisters fall to Yibum to two brothers and one of the sisters is an Ervah to one brother and the other sister is an Ervah to the other brother, each brother may do Yibum to the sister who is not an Ervah.
If two sisters fall to Yibum to two brothers and one of the sisters is an Isur Mitzvah (prohibited mid'Rabanan) to one of the brothers and the other sister is an Isur Mitzvah to the other brother, the brothers may do only Chalitzah and not Yibum. (3)
Rebbi Chiya states that regarding all 15 Arayos mentioned in the Mishnah, there is a possible scenario that two sisters will fall to Yibum from two brothers who died without children, and one of the sisters is an Ervah to one surviving brother, and the other sister is the very same Ervah to the other surviving brother.
Regarding the Ervah of a daughter, this scenario is possible only if the daughter was born out of wedlock, because if she was born from a legitimate relationship it would be impossible for her to have a sister fathered legitimately by the second brother. The reason is that once one brother marries a woman, that woman is forbidden to the other brother even after she is divorced.
According to Rebbi Shimon, if a brother is born after a Yevamah already did Yibum with one of the other brothers, that Yevamah is permitted to the newly-born brother if her brother dies without children. Only according to this opinion is it possible to have a scenario in which two sisters fall to Yibum from two brothers who died without children and one of the sisters is the Ervah of "Eshes Achiv she'Lo Hayah b'Olamo," and the other sister is the very same Ervah to the other surviving brother. (4)
If two brothers are married to two sisters, or to a mother and daughter, or to a mother and granddaughter, and both brothers die, the surviving brother does Chalitzah but not Yibum to the two wives.
Rebbi Shimon derives from a verse that when two relatives fall to Yibum together from two brothers, the Mitzvah of Chalitzah does not apply. However, even according to Rebbi Shimon, if one of the sisters is an Ervah to one of the brothers, he may do Yibum to the other sister.
A BIT MORE
1. If the sister who is the Ervah fell to Yibum first, both brothers are permitted to do Yibum. The brother who is an Ervah to one of the sisters may do Yibum with the sister who is not an Ervah, and the other brother may subsequently do Yibum with the sister who is an Ervah to his brother, because she has the status of a Yevamah who was permitted at first, became prohibited, and then became permitted again.
2. Even though one of the brothers is forbidden to one of the sisters mid'Rabanan, the permitted sister is still regarded as the sister of a Zekukah since the sister is prohibited only mid'Rabanan.
3. Even though, in this case, each one of the brothers is forbidden to one of the sisters, one may have thought that the Rabanan would allow them to do Yibum to the sister who is permitted, since any possible decree does not apply in this case. Even so, the Gemara teaches that Yibum is prohibited.
4. According to Rebbi Shimon, this scenario is possible when there are six brothers. Reuven and Shimon marry two sisters named Leah and Rachel. Reuven dies without children, and subsequently Yisachar is born. After the birth of Yisachar, Levi does Yibum with Leah. Shimon then dies without children, and Zevulun is born. After the birth of Zevulun, Yehudah does Yibum with Rachel. Levi and Yehudah then both die without children. The wife of Levi is an Ervah of "Eshes Achiv she'Lo Hayah b'Olamo" to Yisachar but is permitted to Zevulun since Zevulun was born after Levi did Yibum with her. The wife of Yehudah is prohibited to Zevulun as an Ervah of "Eshes Achiv she'Lo Hayah b'Olamo" but is permitted to Yehudah. This scenario is not possible according the Rabanan who argue with Rebbi Shimon, because they maintain that in this case the wife of Levi is also prohibited to Zevulun even though he was born after the Yibum of Levi. Therefore, Zevulun is prohibited to both the wife of Levi and the wife of Yehudah.
THE CONCERN FOR THE POSSIBILITY OF DEATH
Rebbi Yochanan maintains that we do not suspect that someone will die, and therefore we are not concerned for the cancellation of the Mitzvah of Yibum. The Rashba asks that the Mishnah in Yoma teaches that we are concerned that the wife of the Kohen Gadol will die, and for that reason we set aside another wife for the Kohen Gadol prior to Yom ha'Kipurim. The Rashba answers that we do not have a real concern that she will die, but since the possibility of death always exists, there is nothing wrong with providing for a contingency just in case. However, in this case we will not take away the Mitzvah of Yibum just in case one of the brothers will die, since we do not have a real concern for death.
A YEVAMAH WHO IS PROHIBITED FOR THE FIRST MOMENT
Two brothers were married to two sisters. Both brothers died. But we do not know which one died first. Since it is impossible to do Yibum to both of the sisters and the Zikah applies to both of them, the brother must do Chalitzah and not Yibum. Even if one of the sisters is a Sheniyah to the Yavam or is an Isur Aseh or Isur Lav, the Yavam must do Chalitzah and not Yibum. However, if one of the sisters is an Ervah, the Yavam may do Chalitzah with the other sister since the Zikah does not apply to the Ervah. (Shulchan Aruch EH 175:2)
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