Responsa for Yevamot 61:5
וכי תימא ה"נ לחומרא חומרא דאתי לידי קולא הוא שאם אתה אומר חולצת מתייבמת גירושין דשכיחי גזרו בהו רבנן מפולת דלא שכיחי לא גזרו בהו רבנן
and upon his brother's daughter.<span class="x" onmousemove="('comment',' To whom he had been married. ');"><sup>12</sup></span> and it is not known which of them had died first, her rival must perform <i>halizah</i><span class="x" onmousemove="('comment',' With the daughter's father, the brother of the deceased. Though the dead woman was his forbidden relative, since it is possible that she had been killed before the man, her rival becomes subject to the obligation of performing halizah. ');"><sup>13</sup></span> but may not contract the levirate marriage.<span class="x" onmousemove="('comment',' Infra 67b. Since it is also possible that the man was killed first and the rival remained forbidden to the levir as the rival of his daughter. ');"><sup>14</sup></span>