If brethren dwell together, upon the same paternal inheritance, and one of them die and have no child, no one to perpetuate his family, and thus to keep his property in the possession of the family, the wife of the dead shall not marry without unto a stranger, any one outside her tribe or kindred; her husband's brother, or, as history shows, her nearest kinsman, Genesis 38:8; Ruth 1:12, shall go in unto her and take her to him to wife, and perform the duty of an husband's brother unto her. Thus the marriage with a sister-in-law, otherwise forbidden by Law, Leviticus 18:16, was made a duty in this case.
Related Commentaries of Deuteronomy 25:5
Deuteronomy 25:5
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.