We see a great deal of strife thought out the book of Genesis in the family dynamic of the chosen family of God. For various reasons the firstborn is rejected.
Issac and Ishmael
And Abraham said to God, “O that Ishmael might live in thy sight!” Gen. 17:18
Jacob and Esau
Jacob said to his father, “I am Esau your first-born. I have done as you told me; now sit up and eat of my game, that you may bless me.” Gen. 27:19
And you shall say to Pharaoh, ‘Thus says the LORD, Israel is my first-born son, Ex. 4:22
This is interesting for it is exactly what happened with Jacob’s children. However it was not because of how Jacob felt about them but rather because of their rash actions. Reuben the first born slept with his father’s concubine which lost him the right of the firstborn. Next came Simeon and Levi who for the sake of their sister Dinah killed all the males of Shechem. So the scepter passed to Judah.
3 Reuben, you are my first-born, my might, and the first fruits of my strength, pre-eminent in pride and pre-eminent in power. 4 Unstable as water, you shall not have pre-eminence because you went up to your father’s bed; then you defiled it — you went up to my couch! 5 Simeon and Levi are brothers; weapons of violence are their swords. 6 O my soul, come not into their council; O my spirit, be not joined to their company; for in their anger they slay men, and in their wantonness they hamstring oxen. 7 Cursed be their anger, for it is fierce; and their wrath, for it is cruel! I will divide them in Jacob and scatter them in Israel. 8 Judah, your brothers shall praise you; your hand shall be on the neck of your enemies; your father’s sons shall bow down before you. 9 Judah is a lion’s whelp; from the prey, my son, you have gone up. He stooped down, he couched as a lion, and as a lioness; who dares rouse him up? 10 The scepter shall not depart from Judah, nor the ruler’s staff from between his feet, until he comes to whom it belongs; and to him shall be the obedience of the peoples. 11 Binding his foal to the vine and his ass’s colt to the choice vine, he washes his garments in wine and his vesture in the blood of grapes; 12 his eyes shall be red with wine, and his teeth white with milk. Gen 49:3-12
Manasseh and Ephraim
Joseph called the name of the first-born Manasseh, “For,” he said, “God has made me forget all my hardship and all my father’s house.” Gen. 41:51
3 And Joseph took them both, Ephraim in his right hand toward Israel’s left hand, and Manasseh in his left hand toward Israel’s right hand, and brought them near him. 14 And Israel stretched out his right hand and laid it upon the head of Ephraim, who was the younger, and his left hand upon the head of Manasseh, crossing his hands, for Manasseh was the first-born. 15 And he blessed Joseph, and said, “The God before whom my fathers Abraham and Isaac walked, the God who has led me all my life long to this day, 16 the angel who has redeemed me from all evil, bless the lads; and in them let my name be perpetuated, and the name of my fathers Abraham and Isaac; and let them grow into a multitude in the midst of the earth.” Gen. 48:13-16
The Law of the Firstborn
We don’t find out what all these special rights are until Deuteronomy.
. . . 16 then on the day when he assigns his possessions as an inheritance to his sons, he may not treat the son of the loved as the first-born in preference to the son of the disliked, who is the first-born, 17 but he shall acknowledge the first-born, the son of the disliked, by giving him a double portion of all that he has, for he is the first issue of his strength; the right of the first-born is his. Deut. 21:16-17
No matter what a father feels about his children’s mother the rights of the firstborn are not to be abridged.