TT #676 · 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 8b.
TT lemma: וְיִבְּמָהּ.
וְיִבְּמָהּ – מִצְוָה בּוֹ וְלֹא בְּצָרָתָהּ.
English: “And he shall perform the duty of a husband’s brother for her” — the mitzvah is with her, not with her co-wife.
On the daf / in context: Final reinforcement that the obligation and the permission attach only to the primary yevamah, not to co-wives in cases of ervah.
Derash restricting the mitzvah to the primary yevamah.
The mitzvah of yibbum applies to the yevamah herself and does not extend to her co-wife when the primary woman is an ervah.