6 And it shall be that the firstborn son which she bears will succeed to the name of his dead brother, that his name may not be blotted out of Israel.

Matthew Henry's Commentary on Deuteronomy 25:6

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.