16 then it hath been, in the day of his causing his sons to inherit that which he hath, he is not able to declare first-born the son of the loved one, in the face of the son of the hated one—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.