Yevamot, Chapter Fourteen, Mishnah Four
This mishnah is similar to yesterdays mishnah, the only difference being that in this case, one marriage is of deoraita status, whereas yesterday both were derabbanan.
1) If two brothers, one of whom was deaf and the other of sound senses, were married to two sisters who were of sound senses, and the deaf brother, the husband of the sister who was of sound senses, died, what should the brother of sound senses, the husband of the sister of sound senses, do?
a) [Nothing, since she] is exempt, because she is his wifes sister.
2) If the brother of sound senses, the husband of [the sister who was] of sound senses, died, what should the deaf brother, the husband of [the sister who was] of sound senses, do?
a) He must release his wife with a get, while his brothers wife is forbidden forever [to marry again].
Section one: In this case, when the deaf brother, who was the husband of the sister of sound senses, dies, his wife becomes liable for yibbum or halitzah with the husband of sound senses, who is married to her sister, who is also of sound senses. In this case the deaf widow is not obligated for yibbum or halitzah since her sister is married to the yavam, and that marriage is of deoraita status. As we have learned before, a woman who becomes liable for yibbum with someone prohibited to her is exempt from yibbum.
Section two: This case is opposite. Here the husband of sound senses, married to the woman of sound senses dies, thereby making her liable for yibbum or halitzah with the deaf husband, married to her deaf sister. The deaf husbands wife is now the sister of a woman with whom he is deoraita liable to have yibbum, but his own marriage is only of derabbanan status. Since the connection with his yevamah is of deoraita status, it is strong enough to make his own wife forbidden to him (one cannot marry the wife of his shomeret yavam, a woman with whom is liable to have yibbum). He must therefore divorce his deaf wife. His yevamah can never remarry because she cannot have yibbum or halitzah. She cannot have halitzah because he is deaf. He cannot have yibbum with her because he was derabbanan married to her sister and a man may not marry his wifes sister, even after divorce.