Midrash for Yevamot 26:14

לאפוקי דרבי יוחנן בן נורי דאמר בואו ונתקן להם לצרות שיהו חולצות ולא מתייבמות קמ"ל דבית הלל מכשירים:

afterwards, since the prohibition of marrying a wife's sister<span class="x" onmousemove="('comment',' Which arose later. ');">³⁹</span> could not come and take effect on the prohibition of marrying a brother's wife;<span class="x" onmousemove="('comment',' As legally the widow is only 'his brother's wife' but not 'his wife's sister', her rivals may justly be regarded as strangers who are permitted. ');">⁴⁰</span> where, however, the surviving brother had married first<span class="x" onmousemove="('comment',' And his wife's sister has in consequence become forbidden to him. ');">⁴¹</span>

Sifrei Devarim