15 If a man has two wives, one of them loved and the other disliked, and if both the loved and the disliked have borne him sons, the firstborn being the son of the one who is disliked,

Matthew Henry's Commentary on Deuteronomy 21:15

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.