Reference for Yevamot 34:15

ורב יהודה אמר שומרת יבם שמתה אסור באמה אלמא קסבר יש זיקה ולימא הלכה כדברי האומר יש זיקה

hence he taught us that only after death and not when she is still alive, because it is forbidden to abolish the commandment of levirate marriages. We learned, 'If his deceased brother's wife died he may marry her sister',<span class="x" onmousemove="('comment',' Infra 49a. ');">⁴⁷</span> which implies that her sister only may be married but not her mother! — The same law applies even to her mother; only because he taught in the earlier clause 'if his wife died he is permitted to marry her sister' in which case only her sister is meant and not her mother, since the latter is Biblically prohibited, he also taught in the latter clause 'he is permitted to marry her sister'.<span class="x" onmousemove="('comment',' Her mother, however, is equally permitted. ');">⁴⁸</span> Rab Judah, however, said: If a woman awaiting the decision of the levir<span class="x" onmousemove="('comment',' V. supra, p. 97' n. 11. ');">⁴⁹</span>

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