5 If brothers are living together and one of them, at his death, has no son, the wife of the dead man is not to be married outside the family to another man: let her husband's brother go in to her and make her his wife, doing as it is right for a brother-in-law to do.
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.