16 then it shall be, in the day that he causes his sons to inherit that which he has, that he may not make the son of the beloved the firstborn before the son of the hated, who is the firstborn: 17 but he shall acknowledge the firstborn, the son of the hated, by giving him a double portion of all that he has; for he is the beginning of his strength; the right of the firstborn is his.
Matthew Henry's Commentary on Deuteronomy 21:16-17
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.