Reference for Yevamot 51:21

לעולם אימא לך אין זיקה ומשום דקסבר אסור לבטל מצות יבמין דלמא אדמייבם חד מיית אידך וקמבטל מצות יבמין

HE IS FORBIDDEN TO MARRY HER BUT MAY MARRY HER SISTER,<span class="x" onmousemove="('comment',' Who is not forbidden on account of her rival since the latter is biblically forbidden to the levir and cannot be regarded as his zekukah (v. Glos.). ');">⁵⁰</span> WHILE TO THE SECOND BROTHER BOTH ARE FORBIDDEN. [IF ONE SISTER]<span class="x" onmousemove="('comment',' In the case mentioned in the first paragraph of our Mishnah. ');">⁴⁸</span>

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