Deuteronomy Chapter 25, Verse 5

5: 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.- edit KJV text
5: When brethren dwell together, and one of them dieth without children, the wife of the deceased shall not marry to another: but his brother shall take her, and raise up seed for his brother:— edit Douay text
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.— edit LXX text