I'm glad for the happy ending for Emily and her stepdad. This family sounds lovely, with one bad seed. I know so many similar families.
I think it can be seen as a form of birth defect, in which the person cannot get out of their narcissistic stage, even though they were raised by the same people, in the same environment.
Apparently there's a complex construction in the brain which translates emotion into proper action. This is a level of cause-and-effect inability with the CA's brain. She just FEELS and then she wants control over her feelings which is normal in a toddler who is still learning. Applying it in adult situations is horrifying.
Why do some learn and others don't? Even in a fundamentalist family of today, some just don't believe it and by their teens they are plotting escape.
And some don't. CA might have risen to having her own cult, but doesn't have the people skills.
Also, I suspect cults are a lot of work to get off the ground, then manage. Which is why so many turn their compounds into concentration camps.