Yevamot

he also has the power to impose the obligation of the levirate marriage. AND IS DEEMED TO BE HIS BROTHER IN EVERY RESPECT. In respect of what, in actual practice? — That he is to be his heir and that he  may defile himself for him. Is not this obvious, he being his brother! — Whereas it is written, Except for his kin, that is near unto him,  and a Master had said that 'his kin' refers to his wife, while [on the other hand] it is written, A husband among his people shall not defile himself, to profane himself,  [which verses taken together mean],  some kind of husband may defile himself and some kind of husband may not, and how [is this to be understood]? He may defile himself for his lawful wife but may not defile himself for his unlawful wife; and so here it might have been assumed that he may defile himself for a legitimate brother but may not defile himself for an illegitimate brother; hence it was taught [that it is hot so]. Might we still suggest that it is so? In that case she is liable at any moment to be sent away,  but here he is his brother. FROM THIS IS EXCLUDED A BROTHER BORN FROM A SLAVE OR A HEATHEN. What is the reason? Scripture stated, The wife and her children shall be the master's. IF ONE HAS ANY KIND OF SON, [THAT SON] EXEMPTS etc. What does ANY KIND include? — Rab Judah said: It includes a bastard. What is the reason? — Because Scripture stated, And have no [en lo] child  which implies 'hold an inquiry  concerning him.' AND IS LIABLE TO PUNISHMENT FOR STRIKING [HIM]. But why? One should apply here the Scriptural text, Nor curse a ruler of thy people.  only when he practises the deeds of thy people!  — As R. Phinehas in the name of R. Papa said [elsewhere] 'When he repented', so here also it is a case where he repented. Is such a persona however, capable of penitence? Surely we learnt: Simeon b. Menasya said, That which is crooked cannot be made straight.  refers to him who had intercourse with a forbidden relative and begot from her a bastard! — Now, at any rate. he is practising 'the deeds of thy people'. Our Rabbis taught: He who has intercourse with his sister who is also the daughter of his father's wife  is guilty  on account of both his sister and his father's wife's daughter. R. Jose son of R. Judah said: He is only guilty on account of his sister but not of the daughter of his father's wife. What is the Rabbis' reason? Observe, they would say, it is written, The nakedness of thy sister, the daughter of thy father, or the daughter of thy mother,  what need was there for The nakedness of thy father's wife's daughter, begotten of thy father, she is thy sister?  In order to intimate that he is guilty on account of both his sister and his father's wife's daughter. And R. Jose son of R. Judah? — Scripture stated, She is thy sister,  you can hold him guilty on account of his sister, but you cannot hold him guilty for his father's wife's daughter. And to what do the Rabbis apply the expression, 'She is thy sister'? — They require it [for the deduction] that a man is guilty on account of his sister who is the daughter of his father and the daughter of his mother,  thus indicating that no prohibition  may be deduced by logical argument.  And R. Jose son of R. Judah?  -If so, the All Merciful should have written 'thy sister', what need was there for 'she is'? To indicate that you may hold him guilty on account of 'thy sister' but you cannot hold him guilty on account of 'his father's wife's daughter'. And the Rabbis? Although 'thy sister' was written, It was also necessary to write 'she is'; in order that no one should suggest that elsewhere a prohibition may be deduced by logical argument and that the All Merciful has written here, 'thy sister  because Scripture takes the trouble to write down any law that may be deduced a minori ad majus; hence did the All Merciful write 'she is'. And R. Jose son of R. Judah? — If so, the All Merciful should have written [the expression], 'She is 'thy sister' in the other verse. And to what does R. Jose son of R. Judah apply the phrase Thy father's wife's daughter?  — He requires it for [the deduction]: Only she with whom your father can enter Into marital relationship, but a sister born from a slave or a heathen  is excluded, since your father cannot enter with her into marital relationship. Might it not be said to exclude a sister born from one whom his father had outraged? — You cannot say this owing to Raba's statement. For Raba pointed out a contradiction: It is written In Scripture, The nakedness of thy son's daughter, or of thy daughter's daughter, even their nakedness thou shalt not uncover,  thus it follows that her  son's daughter and her daughter's daughter are permitted; but [below] it is written, Thou shalt not uncover the nakedness of a woman and her daughter; [thou shalt not take] her son's daughter or her daughter's daughter.  How then [are these to be reconciled]? The one  refers to a case of outrage,  the other  to that of lawful marriage.