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, 16 then on the day when he wills his possessions to his sons, he is not permitted to treat the son of the loved as the firstborn in preference to the son of the disliked, who is the firstborn. 17 He must acknowledge as firstborn the son of the one who is disliked, giving him a double portion of all that he has; since he is the first issue of his virility, the right of the firstborn is his.
Matthew Henry's Commentary on Deuteronomy 21:15-17
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.