16 at the time he divides the inheritance with his sons he must not treat the son of the loved wife as the firstborn, cutting out the son of the hated wife, who is the actual firstborn.

Matthew Henry's Commentary on Deuteronomy 21:16

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.