Yevamot, Chapter Fourteen, Mishnah Three
This mishnah, and the rest of the chapter, deals with various combinations of marriages between deaf people and people of sound senses, and the possibility of yibbum, halitzah and divorce.
1) If two deaf brothers were married to two deaf sisters, or to two sisters who were of sound senses, or to two sisters one of whom was deaf and the other was of sound senses;
a) And so also if two deaf sisters were married to two brothers who were of sound senses, or to two deaf brothers, or to two brothers one of whom was deaf and the other of sound senses:
b) Behold these [women] are exempt from halitzah and from yibbum.
2) If [the women] were strangers they must marry them, and if they wish to divorce them, they may do so.
Section one: In all of the cases in this section, there are two marriages between two brothers and two sisters, both of which are between a deaf person and another deaf person or a deaf person and a person of sound senses. In section one both brothers are deaf and in section two both sisters are deaf. In such a case the status of both marriages is rabbbinic (derabbanan). When one brother dies, his wife of derabbanan status becomes liable for yibbum with the other brother, who is married to her sister. Since she cannot marry him because he is married to her sister, she is exempt from both yibbum and halitzah. The key to understanding this mishnah is that both marriages are of equal status. The yavams obligation to have yibbum with her is only derabbanan as is his own marriage and therefore the woman is exempt.
Section two: If the women were not related to each other, then there is no problem with yibbum. However, he may not perform halitzah since deaf men and deaf women cannot perform halitzah (see above 12:4). If he wishes, he may marry her and then divorce her. This is possible in this case because the original marriage of the yevamah to her husband was also only derabbanan. Therefore divorce through gestures is possible.