TT #672 · 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 22b.
TT lemma: וּמֵת אַחַד מֵהֶם וּבֵן אֵין לוֹ.
וּבֵן אֵין לוֹ – עַיֵּן עָלָיו אִם יֵשׁ לוֹ זֶרַע מִכָּל מָקוֹם.
English: “And he has no son” — examine him to see whether he has offspring from any source.
On the daf / in context: Any surviving descendant, regardless of which wife, releases the widows.
Expansive reading of the absence of offspring.
A son (or other descendant) from any wife of the deceased exempts all his widows from yibbum.