Why was it shocking in Middle Eastern custom for the elder to serve the younger?
Middle Eastern custom made the firstborn twin the eldest son and rightful heir, so the older normally held precedence and the younger served him. The declaration that the elder would serve the younger reversed that fixed social order, contradicting what Isaac and Rebekah understood and flying in the face of human reason.
Les explains that the statement “the elder shall serve the younger” was contrary to Middle Eastern custom. Even in the case of twins, whichever twin was born first would be the eldest son and the rightful heir of the inheritance. So this prophecy just flew in the face of what Isaac and Rebekah understood—that the younger, the second twin, Jacob, would be the ascendant one and the one through whom the promises would come. Les notes that this is against human reason, but God, in His foreknowledge before those lads were ever born, could see the end from the beginning and knew that Esau would be destitute of faith.