The Egyptian Woman
The key to understanding Genesis 12-36 is to see it as a story about the education of the patriarchs as patriarchs, that is as the founders of a family that will be able to successfully pass on the covenant, generation to generation.
Or, to put it more succinctly:
Genesis 12-36 is about what it takes to make family work.
In Egypt, Abraham gives his wife to Pharaoh. But, the Lord rescues her and Pharaoh sends them on their way. The lesson for Abraham: a wife is not the same thing as a sister.
Now, it’s as if the roles are reversed: Sarah gives Abraham her Egyptian slave Hagar. The Lord permits Abraham and Sarah to make a mess of things, but then steps in and blesses Hagar’s son with Abraham, Ishmael.
Abraham will subsequently learn that a wife is not the same thing as a concubine.
In each episode, Abraham is learning what the Lord requires of him to be the founding patriarch of a people.
For us, I think the lesson is clear: you can’t have God’s ends apart from God’s means. Trusting in God’s promises means trusting that God will bring them about without your having to force them to happen.