Abraham was converted to the covenant via ritual. It was a bargain to be struck, but his offspring were born into it, as were Isaac’s offspring, and Jacob’s. The general rule has been that a wife must be part of the covenant in order to give birth to children who are in the covenant. Since Asenath is not part of the covenant, they must be brought in via ritual.
Doubtless Joseph could have performed this ritual himself. So why did Jacob adopt Joseph’s sons? We come back to the birthright for this. If there’s a revenge against Joseph’s brothers, it’s served up vicariously through this adoption.
Joseph is set for life. As governor of Egypt, he already has more wealth than Jacob. He actually has no need of any kind of inheritance. While the birthright is often conceived of in terms of material inheritance, there is also the future to consider. Jacob’s sons will become the patriarchs of the tribes of Israel. Joseph will not specifically be named as a tribe patriarch, instead, each of his sons will receive that honor. Together they constitute the House of Joseph, but they individually receive the same honor as all of Jacob’s other sons.
In essence, Joseph receives a double share of the birthright through his sons, as each receives a share on par with the other brothers (except Reuben, who gets the original birthright share).
This is a clever way to circumvent the rules (yet again), but maintains the original intended structure of the birthright. It’s fascinating to see that with four generations of children, the youngest receives more benefit than the eldest, and it’s not finished, yet. Jacob has one more trick to play.