Reference for Yevamot 217:13
<big><strong>מתני׳</strong></big> שני אחין נשואין לשתי אחיות קטנות ומת בעלה של אחת מהן הלזו תצא משום אחות אשה וכן שתי חרשות גדולה וקטנה מת בעלה של קטנה תצא הקטנה משום אחות אשה
'either she or her rival performs halizah'!<span class="x" onmousemove="('comment',' How then could it be said supra that, according to R. Eleazar, the rival may contract the levirate marriage? ');"><sup>41</sup></span> — Are you not [in any case obliged to] offer an explanation?<span class="x" onmousemove="('comment',' The statement being obscure, and an explanation being required in any case. ');"><sup>42</sup></span> Explain, then,<span class="x" onmousemove="('comment',' The statement being obscure, and an explanation being required in any case. ');"><sup>42</sup></span> as follows: She performs <i>halizah</i> while her rival may either perform <i>halizah</i> or contract the levirate marriage.