Deuteronomy 25:5

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

If brothers are living together and one of them dies without a son, his widow must not marry outside the family. Her husband`s brother shall take her and marry her and fulfill the duty of a brother-in-law to 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.