Go back to the very first scene of this play. Gloucester is talking with a guest and introduces one of his sons. There was good sport at the making of him, he jokes.
Edmund is standing there. He listens to his father make his birth a joke in front of a stranger. He bows politely.
That is how this play begins. Before the king has his daughters price their love, this scene comes first. The two households come out of the same door.