Rashi on Deuteronomy 25:5/רש"י על דברים כ״ה:ה׳

1כי 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).

2יחדו 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).

3ובן 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).