Yevamot

If she is forbidden by virtue of a commandment or by virtue of holiness she must perform halizah and may not be taken in levirate marriage!  -There  it is a question of one forbidden by virtue of a commandment alone,  but here [it is a case of one] forbidden by virtue of a commandment and [by virtue of] her sister.  Since it might have been assumed that the prohibition by virtue of a commandment shall take the same rank as the prohibition by the law of incest  and [her sister] should, therefore, be taken in levirate marriage, hence we were taught [that the law is not so]. But how could she  possibly be taken in levirate marriage? Since Pentateuchally she  is to submit to him,  he would come in contact with the sister of his zekukah  -It might have been thought that such provision  was made by the Rabbis for the sake of the precept,  hence we were taught [that it was not so]. IF ONE OF THE SISTERS etc. What need was there again for this statement? Surely, it is precisely identical [with the one before]!  For what difference is there whether [a woman is forbidden] to one or to two? — [Both are] required. For had the former only  been stated, it might have been assumed [that the law was applicable there only] because there exists a second brother to indicate the cause,  but not here where there is no second brother to indicate it.  And if the statement had been made here only it might have been assumed on the contrary that both brothers afford proof in regard to each other,  but not in the other case;  [hence both were] required. THIS IS THE CASE CONCERNING WHICH IT HAS BEEN SAID etc. What is the expression, THIS IS intended to exclude?  -To exclude the case [where one sister was forbidden by] Virtue of a commandment to the one [brother]. and [the other sister was forbidden] by virtue of a commandment to the other. But what need was there for this [additional statement]? Surely it is precisely identical [with that mentioned before];  for what difference is there whether it relates to one or to two! — It might have been thought that only where there is the necessity of providing for a preventive measure against a second brother do we not say that the prohibition by a commandment takes the same rank as a prohibition by the law of incest,  but that where there is no necessity to provide against a second brother we do say that in the case of the one brother the prohibition by a commandment is to be given the same force as the prohibition by the law of incest, and that also in the case of the other brother the prohibition by a commandment is to be given the same force as the prohibition by the law of incest, and that the sisters may consequently be taken in levirate marriage; hence we were taught [that such an assumption is not to be made]. Rab Judah said in the name of Rab and so did R. Hiyya teach: In the case of all these  it may happen that she who is forbidden to one brother  may be permitted to the other,  and that her sister who is her sister-in-law may either perform the halizah or be taken in the levirate marriage;  and Rab Judah interpreted it  [as referring to those]  from one's mother-in-law onward but not to the first six categories. What is the reason? Because this  is only possible in the case of a daughter born from a woman who had been outraged.  but not in that of a daughter born from a legal marriage.  [and the author of that Mishnah]  deals only with cases of legal matrimony and not with those of outraged women. Abaye, however, interprets it  as referring also to a daughter from a woman that had been outraged. because, since [the application of Rab's statement]  is quite possible in her case, it matters not whether she was born from a woman who was legally married or from one that had been outraged; but not to the 'wife of a brother who was not his contemporary' since this  is possible only according to the view of R. Simeon  and not according to that of the Rabbis and he  does not deal with any matter which is a subject of controversy. But R. Safra interprets [it  as referring] also to the 'wife of a brother who was not his contemporary', and this  is possible in the case of six brothers in accordance with the view of R. Simeon.  And your mnemonic is, 'died, born, and performed the levirate marriage; died, born, and performed the levirate marriage'.  [Suppose. for instance]. Reuben and Simeon  were married to two sisters, and Levi and Judah were married to two strangers. When Reuben died, Issachar was born and Levi took the widow in levirate marriage. When Simeon died, Zebulun was born and Judah took [the second widow] in levirate marriage. When Levi and Judah subsequently died without issue and their widows fell under the obligation of the levirate marriage before Issachar and Zebulun, she  who is forbidden to the one  is permitted to the other  while she  who is forbidden to the other  is permitted to the first. In the example of 'her sister who is her sister-in-law',  what need was there  for Judah to contract the levirate marriage? Even if Judah did not contract any levirate marriage it is also possible!  — Owing to the rival.  This  satisfactorily explains the case of the rival; what can be said, however, in respect of the rival's rival?  — If, for instance, Gad and Asher also subsequently married them. MISHNAH. IF TWO OF THREE BROTHERS WERE MARRIED TO TWO SISTERS, OR TO A WOMAN AND HER DAUGHTER, OR TO A WOMAN AND HER DAUGHTER'S DAUGHTER, OR TO A WOMAN AND HER SON'S DAUGHTER, BEHOLD, THESE  MUST  PERFORM THE HALIZAH  BUT MAY NOT BE TAKEN IN LEVIRATE MARRIAGE.  R. SIMEON, HOWEVER, EXEMPTS THEM. IF ONE OF THEM  WAS FORBIDDEN TO HIM BY THE LAW OF INCEST, HE IS FORBIDDEN TO MARRY HER BUT IS PERMITTED TO MARRY HER SISTER. IF, HOWEVER, THE PROHIBITION IS DUE TO A COMMANDMENT OR TO HOLINESS, THEY MUST PERFORM THE HALIZAH BUT MAY NOT BE TAKEN IN LEVIRATE MARRIAGE. GEMARA. It was taught: R. Simeon exempts both from the halizah and the levirate marriage. for it is said in the Scriptures, And thou shalt not take a woman to her sister, to be a rival to her:  when they become rivals to one another,  you may not marry even one of them. IF ONE OF THEM WAS etc. What need was there again for this statement? Surely it is the same!  -It was necessary because of the opinion of R. Simeon: As it might have been assumed that, since R. Simeon had said that two sisters were neither to perform halizah nor to be taken in levirate marriage. A preventive measure should be enacted  against two sisters generally.  hence we were taught  [that it was not so]. IF, HOWEVER, THE PROHIBITION IS DUE TO A COMMANDMENT etc.