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If the now-deceased brother married his wife, and after his death the Yavam marries her sister, we would apply the concept of Ho'il v'Ishteri Ishteri -- since the wife was permitted to him at the time of the death of her husband, even after he marries her sister she remains permitted to him. (1)
Ula agrees that if the Metzora became a Ba'al Keri on the night of the eighth day of his Taharah, he would not be allowed to insert his hands into the Azarah. (2)
All of the 15 Arayos are derived through a Hekesh from each other.
The 15 Arayos are derived through a Hekesh from the sister of a wife and not from the wife of the brother, because the Hekesh that is more stringent is the appropriate one to expound. (3)
A Tzarah of an Ervah who is not married to a brother is permitted. (4)
The Ervah of the sister of a wife applies only during the lifetime of the wife. (5)
Our Sages derive from a verse that the Ervah is always forbidden even if she was not married to the brother, while the Tzarah is only forbidden if she was married to the brother.
Once Yibum is performed the Yevamah is no different than any wife and if he wants to divorce her he gives her a Get and he may remarry her after the divorce (if she did not marry anyone else in between).
A Yavam may perform Yibum against the will of the Yevamah.
A BIT MORE
1. However, if the Yavam married the sister prior to the death of the brother, the concept of Ho'il v'Ishteri Ishteri would not apply, since the brother's wife becomes permitted to him only upon the death of the brother.
2. Since the prohibition of a Ba'al Keri entering the Azarah preceded the time that he was to bring the Korban, the concept of Ho'il v'Ishteri Ishteri does not apply.
3. An additional reason for why the Hekesh from the sister of a wife is used and not the Hekesh from the wife of the brother is that only one prohibition applies to the wife of the brother, while all of the other Arayos have two prohibitions.
4. The verse that prohibits the 15 Arayos from Yibum is necessary only in order to permit the Tzaros of an Ervah who are not married to a brother.
5. The prohibition of the sister of a wife applies even after the wife is divorced. After her death, though, it no longer applies.
WHEN DO WE APPLY THE CONCEPT OF HO'IL V'ISHTERI ISHTERI
Rava says that the verse that prohibits the 15 Arayos from Yibum is not needed to prohibit an Ervah because an Aseh does not supersede a Lo Sa'aseh punishable with Kares; it is needed only to prohibit the Tzaros. Tosfos asks, why would we not need a verse to prohibit an Ervah? Since there is a rule of Ho'il v'Ishteri Ishteri, the verse is necessary for a case in which the Yavam marries the sister of the Yevamah after the death of the brother. Tosfos answers that according to Rava, the concept of Ho'il v'Ishteri Ishteri applies only in a case such as the Metzora who is a Ba'al Keri, since the two prohibitions are actually one and the same: the prohibition for someone Tamei to enter the Beis ha'Mikdash. However, in the case of Yibum, there are two separate prohibitions: the sister of one's wife, and the wife of one's brother. These two Arayos are separate prohibitions in the Torah, and thus, according to Rav, the concept of Ho'il v'Ishteri Ishteri does not apply.
A YEVAMAH IS LIKE ANY OTHER WIFE
Once the act of Yibum is performed, the woman is like an ordinary wife. The husband may divorce her with a Get, and he may subsequently remarry her (if she did not marry anyone else in the interim). (Shulchan Aruch EH 168:1)
One might have thought that once the Yavam divorces the Yevamah, the prohibition of Eshes Ach (the wife of one's brother) goes back into effect. Therefore, this Halachah is needed to teach that this is not the case. (Beis Shmuel)
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