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Once the brother performs Yibum, the Yevamah is regarded as his wife in every aspect. (1)
Although Rebbi Shimon permits the Yevamah to the third brother born after the Yibum of the second brother, because by the time he was born she was permitted, that allowance does not apply to a sister from the mother who marries the brother from the father, because a sister is a prohibition that is never permitted.
If two sisters fall to Yibum from two different brothers, and one of them is an Isur Ervah to the Yavam, he may do Yibum with the other sister.
If the Yevamah is an Isur Mitzvah or an Isur Kedushah, the Yavam does Chalitzah but not Yibum.
Isurei Mitzvah refer to the women whom the Rabanan prohibited. Isurei Kedushah refer to the women who are prohibited with a Lav which is not punishable with Kares. Rebbi Yehudah in the Beraisa reverses the two. (2)
Rava says that one who does not uphold the dictates of the Chachamim is regarded not only as a Kadesh, but he is also considered a Rasha.
A Kohen Gadol may not do Yibum with a widow even if she was a widow from Eirusin and not Nisu'in. (3)
Although an Aseh supersedes a Lo Sa'aseh, our Sages derive from a verse that the Aseh of Yibum does not supersede the Lo Sa'aseh of a Kohen Gadol marrying a widow from Eirusin. (4)
If, b'Di'eved, a Yavam performs Yibum with an Isur Mitzvah, the Tzarah is free to remarry.
If a Petzu'a Daka, Kerus Shafchah, or Saris does Yibum, it is a valid Yibum b'Di'eved, but he may not remain married to the Yevamah because of the Lav that prohibits them from marrying a Jewess.
Rava says that a Kohen Gadol may not do Yibum with a widow from Eirusin because of a Gezeirah that the first Bi'ah that is permitted might be followed with a second Bi'ah that is prohibited.
Reish Lakish says that whenever a Mitzvas Aseh is in conflict with a Lo Sa'aseh, if it is possible to do both of them, one is required to do so. If it is not possible to do both, then the Mitzvas Aseh supersedes the Lo Sa'aseh. (5)
A BIT MORE
1. If they want to split up, he gives her a Get and no Chalitzah is necessary, and he may remarry her if he chooses.
2. The women who are prohibited mid'Rabanan are called Isurei Mitzvah, because it is a Mitzvah to listen to the words of the Chachamim. According to Rebbi Yehudah, they are called Isurei Kedushah, because a person must sanctify himself even with things that are permitted to him by the Torah. The women who are prohibited by the Torah are called Isurei Kedushah, because the verse says "Kedoshim Tiheyu." According to Rebbi Yehudah, they are called Isurei Mitzvah because the verse says, "Eleh ha'Mitzvos."
3. A widow from Eirusin is prohibited only by a Lo Sa'aseh, since she is still a Besulah. A widow from Nisu'in is prohibited with a Lo Sa'aseh and an Aseh.
4. The verse teaches that in the case of a Yevamah who is an Isur Lav, only the Mitzvah of Chalitzah applies, not Yibum. However, this verse does not include an Isur Ervah, because in the case of an Isur Ervah a different verse teaches that if the Mitzvah of Chalitzah applies, then the Mitzvah of Yibum also applies.
5. For this reason, the Mitzvas Aseh of Yibum does not supersede a Lo Sa'aseh, because it is possible to do both Mitzvos by doing Chalitzah instead of Yibum.
A MITZVAS ASEH SUPERSEDES A LO SA'ASEH
Rava says that a Kohen Gadol may not do Yibum with a widow from Eirusin because of a Gezeirah that the first Bi'ah that is permitted might be followed with a second Bi'ah that is prohibited. The Ritva asks that every Yibum should be permitted only for the first Bi'ah and not for a second Bi'ah. Since every case of Yibum is prohibited because of the Ervah of Eshes Ach, and the Torah permits it only for the Mitzvah of Yibum, once the Mitzvah has been completed with the first Bi'ah the Yevamah should now be prohibited. The Ritva answers that the Torah permits Yibum completely, and therefore the Ervah of Eshes Ach no longer applies. However, if the Yevamah is prohibited because of something other than Eshes Ach, the Torah does not permit it and thus the Mitzvah of Yibum may be done only by using the concept of Aseh Docheh Lo Sa'aseh. Therefore, when the Mitzvas Aseh is completed, the Yevamah will be prohibited to the Yavam.
ISUR MITZVAH AND ISUR KEDUSHAH
A Yevamah who is prohibited with a Lav, Aseh, or mid'Rabanan to the Yavam may do Chalitzah but not Yibum. Her Tzarah may do either Chalitzah or Yibum. (Shulchan Aruch EH 174:1)
The Gemara says that if the Yavam is an Isur Lav, she is permitted by the Torah to the Yavam for the first Bi'ah, but the Rabanan made a Gezeirah that the first Bi'ah is forbidden lest it be followed with a second Bi'ah that is prohibited. Tosfos says that even the first Bi'ah is only partially permitted, because Yibum is accomplished with a partial Bi'ah. (Beis Shmuel)
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