So my theory: Bella's house is way too swanky for a librarian's salary, which leaves whatever this missing husband does. I've come to the conclusion that Jill is a Royce (since ya'know...rusty), so I think the Royce's are taking care of Bella on the side, possibly though a husband that they set up for her to have because....well to put it bluntly Bella is a sandwich short of a picnic. Bella is also a Royce, possibly even said dead sister, but the family can't really have someone in her condition seen around as a Royce, so Jill and her get along, so they monitor both bella and jill through them getting along with each other, but at the same time, they feed Jill the dead story so she doesn't associate bella with the family.
Super far fetched, I know. I'm still trying to think of other weird little twists that could make bella so messed up.