17 No, he must acknowledge the inheritance rights of the real firstborn, the son of the hated wife, by giving him a double share of the inheritance: that son is the first proof of his virility; the rights of the firstborn belong to him.

Matthew Henry's Commentary on Deuteronomy 21:17

Commentary on Deuteronomy 21:15-17

(Read Deuteronomy 21:15-17)

This law restrains men from disinheriting their eldest sons without just cause. The principle in this case as to children, is still binding to parents; they must give children their right without partiality.