Deuteronomy 25:5

כי ישבו אחים יחדו IF BRETHREN ABIDE TOGETHER, [AND ONE OF THEM DIE … THE WIFE OF THE DEAD SHALL NOT MARRY ABROAD] — This does not mean that they abide in one city but that they have one “abiding” in the world (that they were living at the same time) thus excluding from the operation of this law the wife of one’s brother who never was in his “world” (i.e. a woman may not marry her brother-in-law who was born after her husband’s death) (Sifrei Devarim 288:2; Yevamot 17b). יחדו TOGETHER — means reciprocally associated (מיוחדים) in the law of inheritance (i.e. that they become the heirs of one another), thus excluding from the duty of יבום the brother on the mother’s side (Sifrei Devarim 288:2). ובן אין לו AND HE HAS NO SON — The last two words suggest עין עליו, investigate about him whether he has descendants of any kind — a son, or a daughter, or a son’s son or a son’s daughter, or a daughter’s son or daughter’s daughter (cf. Yevamot 22b).

כִּֽי־יֵשְׁב֨וּ אַחִ֜ים יַחְדָּ֗ו וּמֵ֨ת אַחַ֤ד מֵהֶם֙ וּבֵ֣ן אֵֽין־ל֔וֹ לֹֽא־תִהְיֶ֧ה אֵֽשֶׁת־הַמֵּ֛ת הַח֖וּצָה לְאִ֣ישׁ זָ֑ר יְבָמָהּ֙ יָבֹ֣א עָלֶ֔יהָ וּלְקָחָ֥הּ ל֛וֹ לְאִשָּׁ֖ה וְיִבְּמָֽהּ׃

If brothers dwell together, and one of them die, and have no child, the wife of the dead shall not marry abroad to a stranger: her husband’s brother shall go in to her, and take her to him to wife, and perform the duty of a husband’s brother to her.