Deuteronomy 25:5-6
5 If brethren dwell together, and one of them die, and have no son, the wife of the dead shall not be married without unto a stranger: her husband's brother shall go in unto her, and take her to him to wife, and perform the duty of a husband's brother unto her. 6 And it shall be, that the first-born that she beareth shall succeed in the name of his brother that is dead, that his name be not blotted out of Israel.
The American Standard Version is in the public domain.
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.