Reference for Yevamot 42:5

איבעיא להו אשת אחי האם מן האם מהו אשת אחי האב מן האם ואשת אחי האם מן האב דאיכא צד אב הוא דגזרו רבנן אבל היכא דליכא צד אב לא גזרו בהו רבנן או דלמא ל"ש

but where there is no paternal strain<span class="x" onmousemove="('comment',' As in the case of the wife of a mother's maternal brother, here under discussion. ');">¹¹</span> the Rabbis did not pass any preventive measure, or is there no difference? R. Safra replied: She herself<span class="x" onmousemove="('comment',' The wife of a mother's paternal brother. ');">¹²</span> is forbidden as a preventive measure; shall we come and superimpose a preventive measure upon a preventive measure! Said Raba: Are not others<span class="x" onmousemove="('comment',' Lit., 'all of them'. v. Rashi, a.l. ');">¹³</span>

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