ghost babysitter, spoiled children, I can't remember!!

[ryand]

I'm trying to find the title of this horror book I read when I was a kid. All I remember is the book was about three spoiled siblings (an overweight kid who eats a lot, a brainiac, and a mean little sister) who were tormented by an evil ghost nanny. I know somewhere in the book they find the evil nanny's babysitting agency, but the building is abandoned. Also, I believe, at the end of the book the kids were in the backseat of their parents car and the babysitter somehow appears in the back seat with them. I've been racking my brain to think of the title, but I've come up with zilch. Any help would be much appreciated.