Deuteronomy 25:5

5If 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.

5“When brothers live together, and one of them dies and has no son, the wife of the deceased shall not be married outside the family to a strange man. Her husband’s brother shall have relations with her and take her to himself as his wife, and perform the duty of a husband’s brother to her.

5“When brothers live together and one of them dies and has no son, the wife of the deceased shall not be married outside the family to a strange man. Her husband’s brother shall go in to her and take her to himself as wife and perform the duty of a husband’s brother to her.

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.

5When brothers dwell together and one of them dies without a son, the widow must not marry outside the family. Her husband’s brother is to take her as his wife and fulfill the duty of a brother-in-law for her.25:5 Cited in Matthew 22:24, Mark 12:19, and Luke 20:28

5Suppose two brothers are living near each other. And one of them dies without having a son. Then his widow must not marry anyone outside the family. Her husband’s brother should marry her. That’s what a brother-in-law is supposed to do.