TT #660 · 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 17b / 29b.
TT lemma: יְבָמָהּ יָבֹא עָלֶיהָ.
אָמַר רַב יְהוּדָה אָמַר רַב: כָּל יְבָמָה שֶׁאֵין אֲנִי קוֹרֵא בָּהּ בִּשְׁעַת נְפִילָה יְבָמָהּ יָבֹא עָלֶיהָ, הֲרֵי הִיא כְּאֵשֶׁת אָח שֶׁיֵּשׁ לוֹ בָּנִים וַאֲסוּרָה.
English: Rav Yehudah said in the name of Rav: Any yevamah of whom I cannot say at the moment of the fall “her yavam shall come upon her” is treated as the wife of a brother who has children and is forbidden.
On the daf / in context: Complex cases (e.g., successive deaths involving sisters) in which the zikah never properly attaches render the woman forbidden as a regular brother’s wife.
Derash limiting the application of the verse to cases where the zikah is viable at the moment of the fall.
If at the time of the brother’s death the yavam is already forbidden to the widow, the verse does not apply and she is treated as a regular forbidden brother’s wife.