I'm looking for a book I read a long time ago, but I can't seem to remember the author's name or the title of the book. It's about a girl named Francesca who lives with her aunt and keeps moving houses (that's one of the details I remember). Her aunt then marries a man who is a part of a cult of some sort and tries to convert everyone else. Francesca doesn't like this because she believes she's a free girl and in the end, she escapes with her step-sister.

I understand this sounds incredibly vague and weird, but if anyone knows the title or author, I'd be really happy! Thanks in advance!