If a man has two wives, one loved and the other unloved, and if both the loved and the unloved bear him sons, but the firstborn is the son of the unloved wife:
She conceived again and bore a son, and said, “It means, ‘The Lord heard that I was unloved,’ and therefore he has given me this one also”; so she named him Simeon.
when he comes to bequeath his property to his sons he may not consider as his firstborn the son of the wife he loves, in preference to the son of the wife he does not love, the firstborn.