Deuteronomy 25:5-6

5"If brothers dwell together, and one of them dies and has no son, the wife of the dead man shall not be married outside the family to a stranger. Her husband's brother shall go in to her and take her as his wife and perform the duty of a husband's brother to her.

6And the first son whom she bears shall succeed to the name of his dead brother, that his name may not be blotted out of Israel.