Midrash for Yevamot 26:15

נתייבמו בית הלל פוסלין כו': הא תו למה לי איידי דתנא חלצו תנא נמי נתייבמו

and the deceased married later,<span class="x" onmousemove="('comment',' When the prohibition of a brother's wife arose. ');">⁴²</span> the prohibition of 'wife's sister' was, surely, first!<span class="x" onmousemove="('comment',' And consequently had taken effect; why then are her rivals permitted? This objection is based on the assumption that Raba, in stating the prohibition of marrying a forbidden relative cannot take effect owing to the prohibition of 'brother's wife', was referring only to such prohibitions as are due to a marriage contract, e.g., a wife's sister. ');">⁴³</span> — Since the prohibition of a 'brother's wife' cannot take effect on the prohibition of 'wife's sister', [any of the other widows] is the rival of a forbidden relative to whom<span class="x" onmousemove="('comment',' Lit., 'in the place'. ');">⁴⁴</span>

Sifrei Devarim