TT #647 · 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 Niddah 44a.
TT lemma: כִּי יֵשְׁבוּ אַחִים יַחְדָּו.
תִּינוֹק בֶּן יוֹם אֶחָד זוֹקֵק לְיִבּוּם, דִּכְתִיב כִּי יֵשְׁבוּ אַחִים יַחְדָּו – אַחִים שֶׁהָיְתָה לָהֶם יְשִׁיבָה אַחַת בָּעוֹלָם.
English: A one-day-old infant binds for yibbum, for it is written “When brothers dwell together” — brothers who had even a single day of simultaneous existence in the world.
On the daf / in context: Minimal simultaneous life is sufficient to create the zikah of yibbum.
Derash on the minimal duration of “dwelling together.”
Even a brother who lived only one day while the other brother was alive creates the bond of yibbum.