Tractate Yevamot - Chapter 14 - Mishnah 6

If two brothers, one of whom was deaf and the other of sound senses, were married to two sisters, one of whom was deaf and the other of sound senses, and the deaf brother, the husband of the deaf sister, died, what should the brother who was of sound senses, the husband of the sister who was 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 deaf sister, do? He must release his wife with a get, while his brother’s wife is forever forbidden [to marry again].