Reference for Yevamot 218:12
אמר רב ששת אמינא כי ניים ושכיב רב אמר להא שמעתא דתניא המקדש את הקטנה קידושיה תלויין מאי תלויין לאו כי גדלה גדלי בהדה ואע"ג דלא בעל
or is the reason because he is of the opinion that when a man betroths the sister of his sister-in-law the latter procures her exemption thereby, but thereby only,<span class="x" onmousemove="('comment',' Lit.,'and she goes for herself'. Only by the 'betrothal' (i.e., the cohabitation) that took place when the minor bad attained her majority does the elder procure her exemptions not by the original betrothal of the minor which is ineffective. ');"><sup>32</sup></span> [and consequently] only if cohabitation has taken place is the elder sister exempt,<span class="x" onmousemove="('comment',' Lit., 'yes'. Because it is the 'betrothal' that severs the levirate bond which existed between the levir and the elder sister from the moment his brother died. ');"><sup>33</sup></span> but if no cohabitation has taken place she is not? — The other replied, This is R. Gamaliel's reason: Because he is of the opinion that when a man betroths the sister of his sister-in-law the latter procures her exemption thereby but thereby only<span class="x" onmousemove="('comment',' Lit.,'and she goes for herself'. Only by the 'betrothal' (i.e., the cohabitation) that took place when the minor bad attained her majority does the elder procure her exemptions not by the original betrothal of the minor which is ineffective. ');"><sup>32</sup></span>