5 When brothers are living together and one of them dies without having had a son, the widow of the dead brother shall not marry a stranger from outside the family; her husband's brother is to come to her and marry her and do the brother-in-law's duty by her. 6 The first son that she bears shall be named after her dead husband so his name won't die out in 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.