Jill Scudder

Possible spoilers

This might seem like a dumb question, but is Jill a child who was actually kidnapped by Roy and Georgina?

I got the impression they had kidnapped her to take the place of Arthur (I think that was the name of their baby who Roy had killed) and had brainwashed her over the years, making her think she really was the sister.

I could be totally wrong in my theory.

In any event, I really enjoyed the movie!