Yevamot

in the name of Rab: Whence is it deduced that betrothal with a sister-in-law  is of no validity?  — From the Scriptural text,  The wife of the dead shall not be married  outside unto one who is not of his kin,  there shall be no validity in the betrothal  of her by a stranger.  Samuel, however, stated: Owing to our [intellectual] poverty  it is necessary [that she be given] a letter of divorce; Samuel having been in doubt as to whether the expression,  The wife of the dead shall not be,  served the purpose of  a negative precept  or rather indicated  that betrothal with such a woman is invalid. R. Mari b. Rachel said to R. Ashi: Thus said Amemar, 'The law is in agreement with Samuel'. Said R. Ashi: Now that Amemar has said that the law is in agreement with Samuel, her levir,  if he was a priest,  submits to her halizah and she is permitted to her second husband.  He  surely benefits  thereby.  and thus the sinner  is at an advantage!  — Rather [this is the reading]: If her levir  was an Israelite, the other  gives her a letter of divorce and she is permitted to the levir. R. Giddal stated in the name of R. Hiyya b. Joseph in the name of Rab: While betrothal with a sister-in-law  is invalid, marriage with her is valid.  If betrothal, however, is invalid, marriage also should be invalid! — Read: Both betrothal and marriage with her are invalid. And if you prefer I might say. What is meant by 'marriage with her is valid'? — It constitutes an act of  harlotry  in accordance with the ruling of R. Hamnuna.For R. Hamnuna stated: A woman who, while awaiting the decision of the levir, played the harlot, is forbidden to marry the levir. And if you prefer I might say: [The reading is]. in fact, as has been originally stated, that betrothal with her is invalid but marriage with her is valid,  since her case might be mistaken for that of a woman whose husband went to a country beyond the sea. R. Jannai said: A vote was taken at the college and it was decided that betrothal with a sister-in-law  has no validity. Said R. Johanan to him: O Master, is not this [law contained in] a  Mishnah? For we have learnt: If a man said to a woman, 'Be thou  betrothed unto me after I shall have become a proselyte'. 'after thou shalt have been a proselyte'. 'after I shall have been emancipated'. 'after thou shalt have been emancipated'. 'after thy husband shall have died', 'after thy sister shall have died' or 'after thy brother-in-law shall have submitted to thy halizah', the betrothal is invalid!  — The other replied: Had I not lifted up the sherd, would you have found the pearl beneath it? Resh Lakish said to him:  Had not a great man praised you. I would have told you that the Mishnah [you cited represents the view] of R. Akiba who maintains that betrothal with those who are subject to the penalties of a negative precept is invalid. If [this Mishnah, however, represents the view of] R. Akiba, betrothal [with the sister-in-law]  should be valid where [the stranger] said to her, 'after thy brother-in-law shall have submitted to thy halizah', since R. Akiba has been heard to state that one may transfer possession of that which is not yet in existence;  for we learned: