Deuteronomy 25:5

And if brethren should live together, and one of them should die, and should not have seed, the wife of the deceased shall not marry out [of the family] to a man not related: her husband's brother shall go in to her, and shall take her to himself for a wife, and shall dwell with her.

If brethren dwell together, and one of them die, and have no child, the wife of the dead shall not marry without unto a stranger: her husband’s brother shall go in unto her, and take her to him to wife, and perform the duty of an husband’s brother unto her.

If brothers dwell together, and one of them dies and has no son, the wife of the dead shall not be married outside 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.