Yevamot

his paternal one he may retain. His mother's maternal sister he must divorce. As to her paternal sister, R. Meir said: He must divorce her,  and the Sages said: He may retain her;  R. Meir maintaining that any woman forbidden on account of maternal consanguinity must be divorced, but if on account of paternal consanguinity he may retain her. He is also permitted [to marry] his brother's wife,  and the wife of his father's brother. All other forbidden relatives are also permitted to him, including his father's wife. If [a proselyte]  married a woman and her daughter  she may  retain  one, but must release the other.  In the first instance he may not marry  her.  If his wife died, he is permitted to marry his mother-in-law. Another opinion is that he is forbidden to marry his mother-in-law.  At all events, it was here stated that he is 'permitted [to marry] his brother's wife'; does not [this apply to a woman] whom his brother had married while he was a proselyte! — No; where he married her while he was still an idolater. What [need was there] to state it?  — It might have been assumed that [in the case of a brother's marriage] while he was still an idolater a preventive measure  should be enacted to preclude [the same thing being done] where he is already a proselyte, hence were we taught [that in such a case a brother's wife was permitted]. The Master stated, 'If [a proselyte] married a woman and her daughter, he may retain one but must release the other; in the first instance he may not marry her'. Now, if he must even release her, is there any need [to speak of a prohibition to marry her] from the outset?  — It refers to a previous clause,  and the meaning is this: That [woman]. concerning whom the Rabbis ruled that he  may retain her,  may nevertheless not be married by him from the outset. 'If his wife died he is permitted to marry his mother-in-law. Another opinion is that he is forbidden to marry his mother-inlaw'. One is in agreement with R. Ishmael and the other is in agreement with R. Akiba. He who forbade the marriage agrees with R. Ishmael who stated: A man's mother-in-law after [his wife's] death retains the former prohibitions;  and in respect of a proselyte a preventive measure was enacted.  He, however, who permits the marriage follows R. Akiba who stated that the prohibition [to marry] one's mother-in-law is weakened after [one's wife's] death;  and, consequently, no preventive measure has been enacted by the Rabbis in respect of a proselyte. MISHNAH. IF THE [MALE] CHILDREN OF FIVE WOMEN WERE MIXED UP  AND, WHEN THESE INTERCHANGED CHILDREN GREW UP, THEY TOOK WIVES AND THEN DIED, FOUR  SUBMIT TO HALIZAH FROM ONE [OF THE WIDOWS]  AND ONE  CONTRACTS WITH HER THE LEVIRATE MARRIAGE.  [THEN] HE  AND THREE [BROTHERS]  SUBMIT TO HALIZAH FROM ONE [OTHER OF THE WIDOWS]. AND ONE  CONTRACTS WITH HER  THE LEVIRATE MARRIAGE.  THUS  EVERY ONE [OF THE WIDOWS] PERFORMS HALIZAH FOUR TIMES AND CONTRACTS THE LEVIRATE MARRIAGE ONCE. GEMARA. Only the halizah [must take place  first] and the levirate marriage afterwards; the levirate marriage, however, must not take place first, since, thereby, one  might infringe the prohibition against a sister-in-law's marriage with a stranger. What [was the object of the statement], HE AND THREE [BROTHERS] SUBMIT TO HALIZAH FROM ONE [OTHER OF THE WIDOWS]?  — That it be not suggested that one brother only should contract levirate marriage with all of them. Rather let every brother contract levirate marriage with only one [of the widows], when it is possible his own [sister-in-law] might happen to fall to his lot. Our Rabbis taught:  'If some of them  were brothers  and some were no brothers,  the brothers submit to halizah while those who are no brothers contract the levirate marriage.' What does this exactly mean? — R. Safra replied. It is this that is meant: If some of them  were paternal brothers  and some were [also] maternal brothers,  the maternal brothers submit to halizah  and the paternal brothers may [also] contract levirate marriage.  'If some of them  were priests and some were non-priests, the priests submit to halizah  and those who are non-priests may [also] contract levirate marriage. If some of them  were priests and some maternal brothers, the former as well as the latter submit to halizah but may not contract levirate marriage.'