16 then on the day when he assigns his possessions as an inheritance to his sons, he may not treat the son of the loved as the first-born in preference to the son of the disliked, who is the first-born,

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.