Deuteronomy 25:5-6

BBE(i) 5 If brothers are living together and one of them, at his death, has no son, the wife of the dead man is not to be married outside the family to another man: let her husband's brother go in to her and make her his wife, doing as it is right for a brother-in-law to do. 6 Then the first male child she has will take the rights of the brother who is dead, so that his name may not come to an end in Israel.