Yevamos, chapter fourteen, mishnah 5. This mishnah closely parallels the previous one: two brothers married to two sisters, where one marriage is on a Torah level and the other only on a rabbinic level. The only difference is the identity of the deaf-mute - in the previous mishnah it was the man, while here it is the woman.
The case:
"Shnei achin pik'chin nesu'in shtei achayos" - two brothers of sound mind and full mental capacity who are married to two sisters.
"Achas chereshes v'achas pikachas" - one of the wives is a deaf-mute, and therefore her marriage is valid only on a rabbinic level; the other is of sound mind, and her marriage is valid on a Torah level.
"Meis pikei'ach ba'al chereshes - mah ya'aseh pikei'ach ba'al pikachas?" - If the brother of sound mind who was married to the deaf-mute died, what should the brother of sound mind married to the woman of sound mind do?
The brother who was married to the deaf-mute, whose marriage was only rabbinic, has died, and the deaf-mute now falls to his brother for yibum. But this yevamah is the sister of his own wife, and his wife is married to him on a Torah level. Therefore "teitzei mishum achos ishah" - she goes free and is not subject to yibum, since she is forbidden to him as his wife's sister on a Torah level.
"Meis pikei'ach ba'al pikachas - mah ya'aseh pikei'ach ba'al chereshes?" - If the brother of sound mind who was married to the woman of sound mind died, what should the brother of sound mind married to the deaf-mute do?
Here the difficulty begins. The brother who was married to the woman of sound mind, whose marriage was on a Torah level, has died, and the yevamah falls to yibum on a Torah level. The surviving brother cannot be exempted from her on the grounds that she is his wife's sister, because his deaf-mute wife is not his wife on a Torah level but only rabbinically, and a rabbinic prohibition has no power to cancel an obligation of yibum and a zikah that come from the Torah.
Therefore the mishnah teaches:
"Motzi es ishto b'get" - he must divorce his deaf-mute wife, since she is the sister of a woman to whom he is bound by a zikah stemming from a Torah-level marriage.
"V'es eishes achiv ba'chalitzah" - the yevamah is not left trapped. Even though he cannot perform yibum with her, since on a rabbinic level she is still his wife's sister, he can perform chalitzah with her - for he is of sound mind, and chalitzah is possible for him.
In summary: in this mishnah we learned the reverse case of the previous one - two brothers of sound mind married to two sisters, one a deaf-mute and one of sound mind. When the husband of the deaf-mute dies, the yevamah goes free and is exempt from everything, because she is his wife's sister on a Torah level. But when the husband of the woman of sound mind dies, the rabbinic bond between the surviving brother and his deaf-mute wife cannot push aside a Torah-level zikah of yibum, and therefore he must divorce his wife with a get and release his yevamah through chalitzah.