Yevamot, Chapter Fourteen, Mishnah Seven
1) If two brothers, one of whom was deaf and the other of sound senses, were married to two strangers who were of sound senses, and the deaf brother, the husband of the woman who was of sound senses, died, what should the brother of sound senses, the husband of the woman of sound senses, do?
a) He either performs halitzah or yibbum.
2) If the brother of sound senses, the husband of the woman who was of sound senses, died, what should the deaf brother, the husband of the woman who was of sound senses, do?
a) He must marry her and he may never divorce her.
Section one: In this case the two brothers are married to two women who are not sisters. Therefore there is no problem of one being married to the sister of the yevamah. In the first section, the deaf brother dies. In such a case there is no problem, and the yavam can have halitzah or yibbum with the deaf brothers widow, as is the case in a normal scenario.
Section two: In this case the brother of sound senses dies, leaving his widow liable for yibbum or halitzah with the deaf brother. The deaf brother cannot have halitzah with her, because deaf people cannot perform halitzah. He may have yibbum with her, but he cannot afterwards ever divorce her. This is because his ties to her are deoraita due to her having been his yevamah. His ability to divorce is only derabbanan, and it is not strong enough to sever such a marriage.