Deuteronomy 21:15-17

If a man has two wives,¹ 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, 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.² 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,³ the right of the firstborn is his.

1 Assuming already that two wives is okay.

2 Protecting the right of the firstborn and sort of standing against favoritism.

3 The reasoning that he is the “first issue of his virility” is not exactly something that would hold much weight today.

Take-home: Family order is upheld by ensuring the firstborn receives the inheritance regardless of whether he is from the favored wife or not, helping keep order in the family after the patriarch dies.