Yevamot

it was for the purpose of permitting a rival where the precept  is not applicable. What is the reason?  — Scripture stated, ''aleha',  to indicate that only in the case of 'unto her'  is she  forbidden,  where the other, however, may not, she is permitted. Said Rami b. Hama to Raba: Might it not be suggested  that the forbidden relative  herself is permitted  where the precept  is not applicable? — Is not [such an argument contrary to the principle of inference] a minori ad majus? Being forbidden where the precept  is applicable, would she be permitted where the precept is not applicable? — ['The case of a] rival', the first replied, 'could prove it, since she is forbidden  where the precept  is applicable, and is permitted  where the precept  is not applicable'. 'It is for your sake,' the other replied, 'that Scripture states, In her life-time,  so long as she  lives'.  But is not the expression,  In her life-time,  required for the exclusion [of the prohibition of marriage] after her  death?  — This is deduced from the text, And a woman to her sister.  If [the deduction were only] from the text. 'And a woman to her sister',  it might have been said that if she  was divorced the sister would be permitted, hence it was expressly stated, 'In her life-time.'  So long as she  is alive, even though she has been divorced, [her sister must] not [be married]!  — But, said R. Huna b. Tahlifa in the name of Raba, two Scriptural texts are available; it is written, Thou shalt not take a woman to her sister, to be a rival to her  [implying two],  and it is also written, To uncover her nakedness,  which implies that only one is forbidden; how then [are the two texts to be reconciled]? Where the precept  is applicable both are forbidden;  where the precept  is not applicable she  is forbidden but her rival is permitted. Might not the deduction be reversed: Where the precept  is applicable she  is forbidden but her rival is permitted, but where the precept is not applicable both are forbidden!  — If so, ''aleha' should not have been stated. Said R. Ashi to R. Kahana: Whence is it derived that the expression ''aleha'  indicates prohibition? Is it not possible that it implies permission, and that it is this that the All Merciful meant to imply: Thou shalt not take a woman to her sister, to be a rival to her,  neither herself nor her rival where 'unto her'  is not applicable,  but where 'unto her'  is applicable  both are permitted!  — If so, how could the 'uncovering of the nakedness' of one  be possible? If in the case where the precept  is applicable, both are permitted;  and if where the precept is not applicable both are forbidden! [Reverting to] the [above] text, Rabbi said: Instead of And take, Scripture stated, 'And take her' and instead of 'And perform the duty of a husband's brother', Scripture stated, 'And perform the duty of a husband's brother unto her', in order to prohibit [the levirate marriage of] forbidden relatives and their rivals. Are, then, rivals mentioned here at all? And, furthermore, the law of rivals has been derived from the expression To be her rival!  — The expression To be her rival is employed by Rabbi for R. Simeon's deduction.  Where,  however, is the rival mentioned?  — What he meant is this: If so,  Scripture should have stated, And take; why then did it state, 'And he shall take her'?  To indicate that wherever there are two to be taken,  he  having the choice of marrying whichever he prefers  both are permitted,  but if not,  both are forbidden; And perform the duty of a husband's brother unto her,  indicates that where levirate marriage is applicable there is the rival forbidden, where, however, levirate marriage is not applicable the rival is permitted. As to the Rabbis,  to what do they apply the verse 'And he shall take her'? — They require it for the deduction of R. Jose b. Hanina. For R. Jose b. Hanina said: 'And he shall take her'  teaches that he  may divorce her with a letter of divorce  and that he may remarry her;  And he shall perform the duty of a husband's brother unto her, even against her will.  And Rabbi?  — The law of R. Jose b. Hanina is deduced from To a wife,  and that the marriage may take place against her will is deduced from Her husband's brother shall go in unto her. What does Rabbi do with [the expression], ''aleha'? — He requires it [for another deduction], as we learnt: The Beth din  are under no obligation  unless [they ruled] concerning a prohibition the punishment for which is kareth, if the transgression was wilful, and a sin-offering if the transgression was unwitting; and so it is with the anointed High priest.