TT #643 · 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 24a.
TT lemma: כִּי יֵשְׁבוּ אַחִים.
מִצְוָה בַּגָּדוֹל לְיַבֵּם, וְאִם קָדַם הַקָּטָן וְיִבֵּם זָכָה.
English: The mitzvah rests primarily upon the eldest brother to perform yibbum; yet if a younger brother precedes him and performs yibbum, he has acquired her.
On the daf / in context: Although the preferred fulfillment is by the eldest, a younger brother’s prior act is valid and effective.
Derash establishing priority of the eldest while validating the younger.
The primary obligation of yibbum falls on the eldest brother, but a younger brother’s prior yibbum is valid.