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A marriage with a woman who is an Isur Lav is a valid marriage. Accordingly, if she gives birth to her husband's firstborn son, the child receives an extra portion of the inheritance of his father.
A marriage with a woman who is forbidden with the punishment of Kares is not a valid marriage. However, a girl born from that marriage is regarded as his child, and if her half-brother has relations with her, he is liable.
A sister born from a Nochri woman or from a maidservant is not regarded as a sister at all, since any marriage with a Nochri is not valid. (1)
A maidservant has an advantage over a Nochris because she is obligated in the Mitzvos, but a Nochris has an advantage over a maidservant because her lineage is legitimate.
Ravina maintains that a child born from a Jewish mother and Nochri father is a Yisrael Pasul but is not a Mamzer.
If a man was Mekadesh one of two sisters but is not sure which one he was Mekadesh, he must give a Get to both of them out of doubt. (3)
If the man who was Mekadesh one of two sisters dies, then if he has one brother, that brother must do Chalitzah to both sisters. If he has two brothers, after one of them does Chalitzah to one of the sisters, the other brother may marry the other sister. However, b'Di'eved, if the two brothers went ahead and married the two sisters, they may remain married to their wives. (4)
If two men are Mekadesh two sisters and there is a doubt about which woman each man was Mekadesh, both men must give a Get to each sister. If they die and each one has one brother, each brother must do Chalitzah to both sisters. If one of them has one brother and the other has two brothers, the one brother does Chalitzah to both sisters, and one of the two brothers does Chalitzah and subsequently the other one may do Yibum. (5)
If each man (in the previous case) has two brothers, then after one of the brothers of each man does Chalitzah with one of the sisters, the other brother of each man may do Yibum with the sister who received Chalitzah from the other person's brother, but not with the sister who received Chalitzah from his brother.
A BIT MORE
1. Even though a marriage with her after she becomes a Ger is a valid marriage, after she becomes a Ger she is a completely different person.
2. Therefore, separate verses are needed to teach that both a marriage with a Nochris and a marriage with a maidservant are invalid.
3. Both sisters are prohibited to him because one of them is the Ervah of the sister of his wife and he does not know which one.
4. The first brother cannot do Yibum to one of the sisters prior to the Chalitzah of the other sister, because he may be marrying the sister of his Yevamah, which is forbidden. However, if he does marry her, b'Di'eved he may remain with her once the other brother does Chalitzah or Yibum to the other sister. In contrast, when there is only one brother who marries one of the two sisters, he may not remain with her even b'Di'eved, because he must do Chalitzah to the other sister, and the sister of a Chalutzah is forbidden.
5. The two brothers cannot do Yibum to one of the sisters prior to the Chalitzah of the one brother, because she may be the wife of the other man and he would be marrying a Yevamah that did not receive Chalitzah.
A HATED WIFE
The verse says, "If a person has both a loved and hated wife." The Gemara asks, "Is there such a thing as a wife who is loved or hated by Hashem?" The Gemara explains that her marriage is hated because she is forbidden to her husband with an Isur Lav. The Tosfos ha'Rosh asks that the question of the Gemara implies that if one of the wives indeed is a Rasha and is hated by Hashem, that would be a valid reason not to give her son the rights of a Bechor to inherit an extra portion. Why, then, does the Gemara not say that the hated wife of the verse is referring to a wife who is a Rasha, and even so the verse is teaching that her firstborn son takes an extra portion? He answers that the implication of the verse is that the wife is hated because of the inappropriate marriage, and not for any other reason. Additionally, the fact that she is a Rasha is not a valid reason to deny her firstborn son his rightful portion of the inheritance.
KIDUSHIN WITH A PROHIBITED WOMAN
A person who performs Kidushin with one of the Arayos accomplishes nothing. The marriage has no validity, with the exception of a Nidah. One who performs Kidushin with a Nidah accomplishes a valid Kidushin, but it is not appropriate to do so l'Chatchilah. (Shulchan Aruch EH 44:6)
It is not appropriate to perform Kidushin with a Nidah because it is forbidden to have marital relations with her. According to the Rambam, a Chupah with a Nidah is not valid. An additional reason for why it is not appropriate is the concern that in the process of the Kidushin, he might touch her. (Beis Shmuel)
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