TT #690 · 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 Yerushalmi Yevamot 1:1.
TT lemma: וְהָיָה הַבְּכוֹר.
בַּמֶּה אֲנַן קַיָּמִין? אִם בַּנּוֹלָד, יֹאמַר יָקוּם עַל שֵׁם אֲחִי אָבִיו הַמֵּת; אֶלָּא אִם אֵינוֹ לַנּוֹלָד, תְּנֵהוּ עִנְיָן לַמְיַבֵּם – מִכָּאן שֶׁמִּצְוָה בַּגָּדוֹל לְיַבֵּם.
English: With what are we dealing? If with a child that is born, let it say “he shall succeed to the name of his father’s brother who died.” Rather, if it is not referring to the child that is born, apply it to the yavam — from here that the mitzvah rests upon the eldest to perform yibbum.
On the daf / in context: The Yerushalmi reinforces that “the firstborn” designates the eldest yavam.
Derash confirming priority of the eldest brother.
The primary mitzvah of yibbum is incumbent upon the eldest surviving brother.