A Little help.
The book is about a teenage girl whose sister joins a cult and has to be rescued. I think it starts out by having the younger sister come home and find that the room that she shares with her older sister has been divided in half by tape and all of her sister's possessions are gone — brought to a pawn shop and sold to benefit the cult. So the younger sister joins the cult and rescues her sister.

Weirdly enough, the book is supposed to be funny (I don't remember if it is or not). [...]
For some reason, I associate the name Daniel Pinkwater with the author, but I have looked through ALL of Daniel Pinkwater's books listed on Amazon and none of these match the description.

If someone could help me find this book or remembers it, I would really appreciate it.

Whatever it is, it is not written by Daniel Pinkwater.