Deuteronomy 21:15-16

15“If a man has two wives, the one loved and 1the other 2unloved, and both the loved and the 3unloved have borne him sons, and the firstborn son belongs to the 4unloved,

16then it shall be on the day that he wills what he owns as an inheritance to his sons, he is not allowed to treat the son of the loved wife as the firstborn, 5at the expense of the son of the 6unloved, who actually is the firstborn son.