5 If brothers dwell together, and one of them die, and have no son, the wife of the dead shall not be married outside to a stranger: her husband’s brother shall go in to her, and take her to him as wife, and perform 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.