TT #666 · Parashas Ki Seitzei · Seventh Aliyah · Passuk 25:05
When brothers dwell together and one of them dies and has no son, the wife of the dead shall not be outside to a strange man; her husband’s brother shall come upon her and take her to himself as a wife and perform the duty of a husband’s brother for her.
Torah Temimah points to Yevamot 11a.
TT lemma: וְיִבְּמָהּ.
וְיִבְּמָהּ – בִּמְקוֹם יִבּוּם הוּא דַּאֲסִירָא צָרָה; שֶׁלֹּא בִּמְקוֹם יִבּוּם שַׁרְיָא.
English: “And he shall perform the duty of a husband’s brother for her” — it is only in a context of yibbum that the co-wife is forbidden; outside a context of yibbum she is permitted.
On the daf / in context: The prohibition of the tzarah (co-wife of an ervah) is limited to situations where yibbum is theoretically applicable.
Derash limiting the tzarah prohibition to the yibbum context.
The co-wife of an ervah is forbidden only when the case falls under the laws of yibbum; otherwise she is permitted.