family has friend live with them and they become a group of artists/ cult

[Hannah]

The daughter of the family has a friend over a lot
and if I remember correctly the book is told from her point of view. The adult friend of the family is a guy who
becomes a cult leader of sorts and then eventually runs away with the parents daughter. Also a notable scene in my mind (for some reason) is the girls spying on the adults a lot and at some point seeing one of them poop in the garden in the middle of the night

[Rainbowheart]

Does anything on the Cults and Communes in Fiction list look familiar?

[Lobstergirl, au gratin]

Fiction or nonfiction? That should be in your header.

Genre as well should be in your header, e.g. Realistic fiction, fantasy, historical fiction, sci-fi.