Commentary for Yevamot 56:9

מהו דתימא כי לא אמרינן אוקי איסור מצוה במקום איסור ערוה היכא דאיכא למיגזר משום שני אבל היכא דליכא למיגזר משום שני אימא להאי אוקימנא איסור מצוה במקום איסור ערוה ולהאי [אוקימנא] איסור מצוה במקום איסור ערוה ולייבמו קמ"ל

but not here where there is no second brother to indicate it.<span class="x" onmousemove="('comment',' Since both brothers marry respectively the two sisters, it might be assumed that any levir may marry the sister of his zekukah. ');">¹⁴</span> And if the statement had been made here only it might have been assumed on the contrary that both brothers afford proof in regard to each other,<span class="x" onmousemove="('comment',' Since each brother is permitted to marry only one particular sister and not the other, it is obvious that the other is forbidden to him. The law of zekukah could not consequently be mistaken. ');">¹⁵</span>

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