Yevamot

the prohibition depends on the number of days,  and with a brother's wife the All Merciful made her pro hibition dependent on the birth of children!  — But the objection may be raised thus: A menstruant and a brother's wife are different,  since the man who caused them to be forbidden cannot cause them to be permitted.  Would you [then] apply their restrictions to a married woman whose permissibility is brought about  by the man who caused her to be forbidden? But, said R. Johanan, or as some say, R. Huna son of R. Joshua, Scripture stated, For whosoever shall do any of these abominations, even the souls that do them shall be cut off,  all forbidden unions were compared to the menstruant;  as the first stage constitutes the offence with the menstruant so does the first stage constitute the offence with all the others. What need, then, was there  to mention the menstruant in the context of brother's wife?  — For an inference like that of R. Huna. For R. Huna stated: Whence in the Torah may an allusion to the sister-in-law  be traced? [You ask,] 'Whence'? Surely it is written in Scripture, Her husband's brother shall go in unto her!  — [The query is] rather, whence the allusion that a sister-in-law is forbidden  during the lifetime of her husband?  But surely this is a logical inference: Since the All Merciful said that she  is permitted to marry after the death of her husband, it may be inferred that during the lifetime of her husband  she is forbidden! — [No] for is it not possible [to maintain] that after the death of her husband it  is a commandment, and during the lifetime of her husband it  is only optional? Or else, [though] indeed,  only after the death of the husband,  and not during the lifetime of her husband; yet being a negative commandment  that is derived from a positive one  it has only the force of a positive commandment!  — Scripture stated: And if a man shall take his brother's wife, she is a menstruant.  Now is a brother's wife always a menstruant?  But the meaning is, 'like a menstruant': as a menstruant, although permitted afterwards,  is forbidden under the penalty of kareth during the period of her prohibition, so also a brother's wife, though permitted afterwards,  is forbidden under the penalty of kareth during the lifetime of her husband. What need, however, was there to mention the first stage in connection with a father's sister or a mother's sister?  — For an inference like that mentioned in the following question which Rabina addressed to Raba: What is the law if a man passed the first stage in pederasty? [You ask,] 'What is the law in pederasty'? Surely it is written, As with womankind!  — But [the query is] what is the law when one passed the first stage with a beast? The other replied: No purpose is served by the text  in [forbidding] the first stage in the case of a father's sister and a mother's sister, since in their case the prohibition is arrived at by the comparison of R. Jonah, apply that text to the first stage with a beast. Observe! Intercourse with a beast is among the offences subject to the death penalties of a Beth din; why then was the first stage in relation to it enumerated among offences that are subject to the penalty of kareth?  It should rather have been written among those which are subject to the death penalty of the Beth din,  and thus one offence that is subject to the death penalty of a court would be inferred  from a similar offence that is subject to the death penalty of a court! — Since the entire context  was to serve the purpose of exposition,  this thing  was also included that it may serve the purpose of exposition. What is the exposition?  — It was taught, Thou shalt not uncover the nakedness of thy father's sister,  whether she is paternal or maternal. You say, 'Whether she is paternal or maternal', perhaps it is not so, but only when she is paternal and not when maternal? — This  is only logical: A man is subject to a penalty  in this case and he is also subject to penalty  in the case of his sister; as with his sister it is the same whether she is paternal or maternal, so here also  it is the same whether she is paternal or maternal. But might it not be argued  in this way: A man is subject to a penalty  in this case and is also subject to a penalty  in the case of his aunt;  as his aunt is forbidden only when she is paternal  but not when maternal,  so here also  when she is paternal and not when maternal! — Let us consider whom it more closely resembles. A prohibition which is natural  ought to be inferred from a prohibition which is also natural  but let no proof be adduced from an aunt whose prohibition is not natural.  But might it not be argued  thus: The relatives of a father  should be inferred from the relatives of a father  but let no proof be adduced from a sister who is one's own relative! Hence it was stated,  Thou shalt not uncover the nakedness of thy father's sister,  implying  whether paternal or maternal, and Thou shalt not uncover the nakedness of thy mother's sister,  implying also whether paternal or maternal. What need was there to write it  in respect of a father's sister and also in respect of a mother's sister?  — R. Abbahu replied: Both are required. For had the All Merciful written it  in respect of a father's sister [it might have been assumed to apply to her alone] because her relationship is legally recognized,  but not to a mother's sister.  And had the All Merciful written it  in respect of a mother's sister [it might have been assumed to apply to her alone] because her relationship is certain, but not to her father's sister.  [Hence both were] required. As to one's aunt concerning whom the Tanna had no doubt that she must be paternal  and not maternal, whence does he derive it? Raba replied: It is arrived at by a comparison between the words 'His uncle' [in two passages]: Here it is written, He hath uncovered his uncle's nakedness,  and there it is written, Or his uncle or his uncle's son may redeem him,  as there he must be paternal and not [necessarily] maternal  so here also, he  must be paternal  and not [necessarily] maternal. And whence is it  proved there? — Scripture stated, Of his family may redeem him,  and only a father's family may be called the proper family, but the mother's family cannot be called the proper family. But surely we learned: If a man was told, 'Your wife is dead', and he married her paternal sister; [and when he was told] 'She  also is dead', he married her maternal sister; 'She  too is dead', and he married her paternal sister; 'She  also is dead', and he married her maternal sister, he is permitted  to live with the first,  third  and fifth  who also exempt their rivals;  but he is forbidden to live with the second and the fourth,  and cohabitation with one of these does not exempt her rival. If, however, he cohabited with the second after the death of the first, he is permitted to live with the second  and with the fourth  who also exempt their rivals,  but he is forbidden to live  with the third and with the fifth.