Deuteronomy 25:5-6

5 When brothers reside together, and one of them dies and has no son, the wife of the deceased shall not be married outside the family to a stranger. Her husband's brother shall go in to her, taking her in marriage, and performing the duty of a husband's brother to her,
6 and the firstborn whom she bears shall succeed to the name of the deceased brother, so that his name may not be blotted out of Israel.