Why was the birthright in Abraham's family more than a family priesthood?
It was more than a family priesthood because the birthright identified the line through which the promised Messiah and Redeemer would come. That made it central to God's sovereign will, not merely the eldest son's role as family spiritual leader. Esau forfeited it by unbelief, and it fell to Jacob.
Les explains that the birthright was the spiritual side of the family position: the eldest son was more or less the priest, or spiritual leader, of the family. But in Abraham's family, the birthright was more than that family priesthood. It was the line through which the promised Messiah and Redeemer would come, making it all-important that God's sovereign will be accomplished. Esau, born first, could have been in the Messiah's line, but he canceled it because of his no faith, so it fell to Jacob. Les also notes that Jacob got the birthright, not really by deceiving Esau.