Deuteronomy 25:5-6

5“If brothers are living together and one of them dies without a son, the widow of the deceased shall not be married outside the family to a stranger. Her husband’s brother shall be intimate with her after taking her as his wife and perform the duty of a husband’s brother to her.

6It shall be that her firstborn [son] will be given the name of the dead brother, so that his name will not be blotted out of Israel.