הלכה על יבמות 34:8

From the laws of the commandment is that which they, may their memory blessed, said (Yevamot 17b), "'When brothers dwell together' - to exclude the wife of a brother who was not in his world"; meaning that [then] she does not have an obligation of levirate marriage and not of release - for example, [if] this brother was born after his brother [that was married and had no children] died. And that which they also inferred (Yevamot 17b), "'Brothers together' - that are together in inheritance, to exclude brothers [born only] from the mother," who are not obligated in levirate marriage and release. "'And has no (ein) son' - look into (ayen) him" (Yevamot 22b). [This is] meaning to say that if the brother did have a son or daughter from any place, or the son of a son or the son of a daughter or the daughter of a daughter or any one that came out from his 'thigh' - there is no commandment of levirate marriage or release there, for the reason that we mentioned concerning the commandment of levirate marriage.