Deuteronomy 21:14-17

14But if it happens that you are no longer pleased with her, let her go wherever she wants. You must never sell her or mistreat her as if she were a slave, since you’ve already had sex with her.

15A man might have two wives and love one but not the other. Both wives might have children, and the firstborn son might belong to the wife that the man doesn’t love.

16When the day comes for the father to give his sons their inheritance, he can’t treat the son of the wife he loves as if that son were the firstborn. This would show a total disregard for the real firstborn (the son of the wife he doesn’t love).

17Instead, he must recognize the son of the wife he doesn’t love as the firstborn. He must give that son a double portion of whatever he owns. That son is the very first son he had. The rights of the firstborn son are his.