Reference for Yevamot 47:1

התם אי למ"ד יש זיקה יש זיקה ואי למ"ד אסור לבטל מצות יבמין אסור לבטל מצות יבמין הכא כל חד וחד אימור דידיה קא מתרמיא ליה:

There,<span class="x" onmousemove="('comment',' Where both sisters are bound by the levirate tie. ');">¹</span> if one is to follow the view of him who said that a levirate bond does exist,<span class="x" onmousemove="('comment',' Between the levir and his deceased brother's widow from the moment death took place. ');">²</span> a levirate bond exists;<span class="x" onmousemove="('comment',' Consequently both widows are forbidden in levirate marriage, each being in relation to the other a sister of one's zekukah. But such prohibition is never removed even when one of them subsequently performed the halizah with one of the brothers and has thus severed her levirate bond, for once a yebamah is prohibited to her deceased husband's brother for a single moment, she is in the same category as a widow of a brother who died with issue. ');">³</span>

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