TT #684 · Parashas Ki Seitzei · Seventh Aliyah · Passuk 25:06
And it shall be that the firstborn that she bears shall succeed to the name of his dead brother, so that his name shall not be blotted out from Israel.
Torah Temimah points to Yevamot 24a.
TT lemma: וְהָיָה הַבְּכוֹר.
וְהָיָה הַבְּכוֹר – מִצְוָה בַּגָּדוֹל לְיַבֵּם.
English: “And it shall be that the firstborn” — the mitzvah rests primarily upon the eldest brother to perform yibbum.
On the daf / in context: The verse is read as referring to the yavam himself (the eldest), establishing priority among the brothers.
Derash identifying the primary subject of the mitzvah.
The eldest brother has the primary obligation (and right) to perform yibbum.