Yevamot

From this it clearly follows that a wife's sister, whether she is paternal or maternal, is forbidden.  Whence, however, is this derived? — Deduction is made from one's sister; as a sister [is forbidden] whether she is paternal or maternal, so here also  whether she is paternal or maternal. But let the deduction  be made from one's aunt; as one's aunt [is forbidden only when she is] paternal  and not when maternal, so here  also [the prohibition should apply when she is] paternal and not when maternal! — It stands to reason that the deduction should be made from one's sister, since [laws concerning] his own relatives  [should be inferred] from [laws concerning others of] his own relatives.  On the contrary! Deduction  should have been made from one's aunt, since a relationship effected through betrothal  [should be inferred] from one effected through betrothal!  — The deduction  is rather made from a brother's wife, since her relationship  is through betrothal, and she is of his own relatives. Whence, however, is [the law concerning] a brother's wife herself derived? — From what was taught: Thou shalt not uncover the nakedness of thy brother's wife,  whether he is paternal or maternal. You say, 'Whether he is paternal or maternal', perhaps it is not so, but only when paternal and not when maternal? This is a matter of logical argument: He is subject to a penalty here  and he is also subject to penalty [for intercourse] with his sister; as [the prohibition of] his sister applies whether she is paternal or maternal, so here also  [the prohibition applies] whether he  was paternal or maternal. But might it not be argued  thus: He is subject to a penalty here  and he is also subject to penalty [for intercourse] with his aunt. As therefore [the prohibition of] his aunt applies only when she is paternal  and not when only maternal, so here  also [the prohibition applies only when he  is] paternal and not when only maternal! Let us observe whom the case  more closely resembles. Deduction concerning one's own relatives should be made from one's own relatives, and let no proof be adduced from one's aunt whose relationship is due to his father. But might it not be argued as follows:  Deduction should be made concerning a relationship which is due to betrothal  from a relationship that is due to betrothal,  but let no proof be adduced from a sister the prohibition of whom is natural!  — For this reason  it was specifically stated in Scriptures, It is thy brother's nakedness,  implying  whether he is paternal or only maternal. Might it not be suggested that the one as well as the other  speaks of the wife of a paternal brother, the one referring to a brother's wife who had children during the lifetime of her husband,  while the other refers to a brother's wife who had no children during the lifetime of her husband! — The case of one who had no children during the lifetime of her husband may be deduced from the statement of R. Huna. Might not both  still speak of the wife of a paternal brother, the one referring to a brother's wife who had children during the lifetime of her husband and the other to one who had children after the death of her husband! — The case of one who had children after the death of her husband requires no Scriptural text; for since the All Merciful said that she who had no children was permitted, it is obvious that if she had children she is forbidden. Is it not possible that she who has no children is forbidden to all men but permitted to the levir while she who has children is permitted both to all men and to the levir! Or else: If she has no children it is a commandment  but if she has children it is optional! Or else: [Though indeed] the levir may marry her if  she has no children but he may not if she has children, yet [as the prohibition  is] a negative commandment that is derived from a positive one  it has only the force of a positive commandment!  — For this reason Scripture wrote another text,  He hath uncovered his brother's nakedness.  But might it be said that the wife of a maternal brother is like the wife of a paternal brother, and that as the wife of a paternal brother is permitted  after the death of her husband, so is also the wife of a maternal brother  permitted after the death of her husband! — Scripture said, She is,  she retains her status. What need was there to specify the penalty of kareth for intercourse with one's sister?  — To infer a ruling like that of R. Johanan. For R. Johanan stated: If one committed all these offences  in one state of unawareness, he is liable for every one of them.  According to R. Isaac, however, who stated, 'All those who are subject to the penalty of kareth were included in the general rule; and why was the penalty of kareth for [intercourse with] a sister stated separately? In order to indicate that his  penalty is kareth and not flogging',  whence is the division  deduced? — It is deduced from, And unto a woman … as long as she is impure by her uncleanness,  that guilt is incurred for every single woman. For what purpose did the All Merciful write, They shall be child less  in the case of one's aunt?  — It is required for an exposition like that of Rabbah. For Rabbah pointed out the following contradiction: It is written, They shall be childless,  and it is also written, They shall die childless!  How [are these two versions to be reconciled]? If he has children he will bury them; if he has no children, he will be childless. And it was necessary to write They shall be childless,  and it was also necessary to write, They shall die childless.  For had the All Merciful written only, They shall be childless,  it might have been assumed to refer to children born before the offence  but not to those born subsequent to the offence,  hence the All Merciful wrote, They shall die childless.  And had the All Merciful written, They shall die childless,  it might have been assumed to refer to those born subsequent to the offence,  but not to those who were born previously,  [hence both texts were] required. Whence [is the prohibition of] the first stage among those who are subject to the penalty of negative commandments  to be inferred? — As the All Merciful specified carnally  in the case of a designated  bondmaid,  it may be inferred that among all the others who are subject to the penalty of negative commandments,  the first stage by itself constitutes the offence.  On the contrary! As the All Merciful specified the first stage in the case of those who are subject to the penalty of kareth,  it may be inferred that among those who are subject to the penalty of negative commandments consummation only constitutes the offence! — R. Ashi replied: If so,  Scripture should have omitted [the reference]  in the case of the designated handmaid. Whence [is the prohibition of] the first stage inferred in the case of offences for which priests alone are subject to the penalty of negative commandments?  — This is arrived at by an analogy between the expressions of 'taking'. Whence [is the prohibition  in respect of] those who are subject  to the penalty of a positive commandment  inferred?