Reference for Arakhin 50:14

ותיפוק ליה דהכא תרתי והכא חדא

Surely if there is a son, no levirate duty is involved.<span class="x" onmousemove="('comment',' Any child, son or daughter, of the dead brother renders the levirate duty impossible, and indeed prohibits it as incestuous. Hence the brother plays a role only when there is no son.');">⁶</span>

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