Responsa for Yevamot 214:3

אלמא בשעת נפילה נראית ככלתו הכא נמי בשעת נפילה נראית כצרת בתו

— The rival of a forbidden relative is different.<span class="x" onmousemove="('comment',' From a minor who becomes subject to halizah. While the minor may, by annulling her marriage retrospectively by the exercise of the right of mi'un, procure exemption from the halizah, her rival cannot, through the minor's exercise of this right, obtain the freedom to marry the minor's forbidden relative. ');">⁸</span> For Rami b. Ezekiel learnt: If a minor made a declaration of refusal against her husband she is permitted to marry his father,<span class="x" onmousemove="('comment',' Who, owing to her retrospective annulling by mi'un of her marriage with his son, is to him now a mere stranger. ');">⁹</span> but if against the levir<span class="x" onmousemove="('comment',' To whom she has become bound by the levirate obligation when her husband, against whom she did not exercise her right of mi'un, died childless. ');">¹⁰</span> she is forbidden to marry his father. It is thus evident<span class="x" onmousemove="('comment',' Since she is forbidden to marry the levir's father. ');">¹¹</span>

Teshuvot Maharam