Midrash for Yevamot 26:13

חלצו בית שמאי פוסלים וכו': פשיטא

is that one prohibition<span class="x" onmousemove="('comment',' That, e.g., of marrying a brother's wife. ');">³⁶</span> cannot take effect on another prohibition.<span class="x" onmousemove="('comment',' That of marrying a forbidden relative (e.g., a daughter). Since the latter prohibition takes no effect in such a case, the forbidden relative whom the levirate bond does not consequently affect may be regarded as non-existent, so far as her levirate obligations are concerned. Her rivals, therefore, come under the category of complete strangers and are consequently permitted to the brothers. ');">³⁷</span> This explanation is satisfactory in the case where the deceased had married first and the surviving brother married<span class="x" onmousemove="('comment',' A sister of his brother's wife. ');">³⁸</span>

Sifrei Devarim