Deuteronomy 25:5-6

“If two brothers are living together on the same property and one of them dies without a son, his widow may not be married to anyone from outside the family. Instead, her husband’s brother should marry her and have intercourse with her to fulfill the duties of a brother-in-law. The first son she bears to him will be considered the son of the dead brother, so that his name will not be forgotten in Israel.