Tractate Yevamot - Chapter 14 - Mishnah 4

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? [Nothing, since she] is exempt, because she is his wife’s sister. 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? He must release his wife with a get, while his brother’s wife is forbidden forever [to marry again].