5 When brothers reside together, and one of them dies and has no son, the wife of the deceased shall not be married outside the family to a stranger. Her husband's brother shall go in to her, taking her in marriage, and performing the duty of a husband's brother to her,
Matthew Henry's Commentary on Deuteronomy 25:5
Commentary on Deuteronomy 25:5-12
(Read Deuteronomy 25:5-12)
The custom here regulated seems to have been in the Jewish law in order to keep inheritances distinct; now it is unlawful.