I read a book in elementary school, but I can't remember the name. Here's a few things I can remember: A man (who already has a daughter) gets married to a goth lady with three kids. One of the new step-siblings is a little girl who can seemingly turn into a cat; the middle child is a scientist who does crazy experiments; and the oldest child is thought to be a vampire. The entire book is written from the POV of the man's daughter. In the book, the parents go off on their honeymoon and hire a military nanny who is very strict, and the kids team up to get rid of her.
That's all I can remember.