Deuteronomy 25:5-6

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

6Now the firstborn that she bears is to carry on the name of his dead brother, so that his name will not be blotted out of Israel.