5 If brethren dwell together, and one of them die, and have no son, the wife of the dead shall not marry a stranger abroad: her husband's brother shall go in unto her, and take her to him as wife, and perform the duty of a husband's brother unto her. 6 And it shall be, that the firstborn that she beareth shall stand in the name of his brother who is dead, that his name be not blotted out from Israel.
Matthew Henry's Commentary on Deuteronomy 25:5-6
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.