Yevamot

that he may divorce her with a letter of divorce and that he may remarry her',  let it there also be said, 'And perform the duty of a husband's brother unto her,  the former levirate attachment still remains with her' and, consequently, she should require halizah [also]! — There the case is different; since Scripture stated, 'And take her to him to wife',  as soon as he married her she becomes his wife in every respect. If so, [the same deduction should be applied] here also! — Surely the All Merciful has written, 'And perform the duty of a husband's brother unto her'.  And why the differentiation?  - It stands to reason that permission  should be applied to that which is [also otherwise] permitted,  and that prohibition  should be applied to that which is [also otherwise] prohibited. According to R. Simeon, however, who stated, 'Because when he was born he found her permitted, and she was never forbidden to him even for one moment',  a brother, if this reason is tenable,  should be allowed to take in levirate marriage his maternal sister whom his paternal brother had married prior to his birth, dying subsequently, since, when he was born, he found her permitted.  — Whither did the 'prohibition of sister' vanish?  — Here, also, whither did the prohibition of 'the wife of the brother who was not his contemporary' vanish! — The one  is a prohibition which can never be lifted; the other  is a prohibition which may be lifted. MISHNAH. A GENERAL RULE HAS BEEN LAID DOWN  IN RESPECT OF THE DECEASED BROTHER'S WIFE:  WHEREVER SHE IS PROHIBITED  AS A FORBIDDEN RELATIVE, SHE  MAY NEITHER PERFORM THE HALIZAH NOR BE TAKEN IN LEVIRATE MARRIAGE. IF SHE IS PROHIBITED BY VIRTUE OF A COMMANDMENT  OR BY VIRTUE OF HOLINESS,  SHE MUST PERFORM THE HALIZAH AND MAY NOT BE TAKEN IN LEVIRATE MARRIAGE. IF HER SISTER IS ALSO HER SISTER-IN-LAW,  SHE  MAY PERFORM THE HALIZAH OR MAY BE TAKEN IN LEVIRATE MARRIAGE. PROHIBITED BY VIRTUE OF A COMMANDMENT' [REFERS TO] THE SECONDARY DEGREES IN RELATIONSHIP FORBIDDEN BY THE RULING OF THE SCRIBES. 'PROHIBITED BY VIRTUE OF HOLINESS' [REFERS TO THE FOLLOWING FORBIDDEN CATEGORIES]: A WIDOW TO A HIGH PRIEST;  A DIVORCED WOMAN, OR ONE THAT HAD PERFORMED HALIZAH TO A COMMON PRIEST;  A FEMALE BASTARD OR A NETHINAH  TO AN ISRAELITE;  AND A DAUGHTER OF AN ISRAELITE,  TO A NATHIN  OR A BASTARD. GEMARA. What was the GENERAL RULE meant to include?  — Rafram b. papa replied: TO include the rival of a woman who was incapable of procreation, In agreement with the view of R. Assi. Some there are who say:  'Whenever her prohibition is that of a forbidden relative then only is her rival forbidden; when, however, her prohibition is not that of a forbidden relative, her rival is not forbidden'. What was this meant to exclude? — Rafram replied: To exclude the rival of one incapable of procreation, contrary to the view of R. Assi. IF HER SISTER IS ALSO HER SISTER-IN-LAW [etc.]. Whose sister? If the sister of her who is forbidden by Virtue of an ordinance of the Scribes be suggested, fit may be objected,] since, pentateuchally, she  is subject to the levir, he would  come in marital contact with the sister of her who is connected with him by the levirate bond! — It means the sister of her who is prohibited to him as a forbidden relative. PROHIBITED BY VIRTUE OF A COMMANDMENT', [REFERS TO] THE SECONDARY DEGREES. Why are these designated, PROHIBITED BY VIRTUE OF A COMMANDMENT'? — Abaye replied: Because it is a commandment to obey the rulings of the Sages. PROHIBITED BY VIRTUE OF HOLINESS' … A WIDOW TO A HIGH PRIEST; A DIVORCED WOMAN, OR ONE WHO HAD PERFORMED THE HALIZAH, TO A COMMON PRIEST. Why are these designated 'PROHIBITED BY VIRTUE OF HOLINESS'? — Because It is written in the Scriptures, They shall be holy onto their God. It was taught: R. Judah reverses the order: prohibited by virtue of a commandment [refers to the following prohibited categories:] a widow to a high priest; a divorced woman or one that had performed halizah, to a common priest. And why are these designated, prohibited by virtue of a commandment? — Because it is written in the Scriptures, These are the commandments.  prohibited by virtue of holiness [refers to] the secondary degrees of relationship forbidden by the rulings of the scribes. And why are these designated, prohibited by virtue of holiness? — Abaye replied: Because whosoever acts in accordance with the rulings of the Rabbis is called a holy man. Said Raba to him: Then he who does not act in accordance with the rulings of the Rabbis is not called a holy man; nor is he called a wicked man either?  — No, said Raba: 'Sanctify yourself by that which is permitted to you'. A WIDOW TO A HIGH PRIEST. An unqualified ruling is laid down making no distinction between a nissu'in  widow and an erusin  widow. Now, one can well understand the reason the case of a nissu'in widow [since marriage with her is forbidden by] a positive  and a negative precept,  and no positive precept  may override both a negative and a positive precept. In the case, however, of an erusin widow [marriage with whom is forbidden by] a negative precept only,  let the positive precept  override the negative  one? — R. Giddal replied in the name of Rab: Scripture stated, Then his brother's wife shall go up to the 'gate,  where there was no need to state his brother's wife;  why then was 'his brother's wife' specified? [To indicate that] there is a case of another  brother's wife who goes up for halizah but does not go up for levirate marriage.  And who is she? One of those prohibited  by a negative precept. Might it  not be said [to include also] such as are subject to the penalty of kareth?  — Scripture said, If the man like not to take,  if he likes, however, he may take her in levirate marriage, [hence it is to be inferred that] whosoever may go up to enter into levirate marriage may also go up to perform halizah and whosoever may not go up to enter into levirate marriage  may not go up to perform halizah either. If so, the same should apply also to those forbidden by a negative Precept! — But, surely, the All Merciful has included them [by the expression] 'His brother's wife'. What ground is there for such differentiation?