5 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.

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.