OP, I'm sorry you're going through this. Add me to the list of people you should feel free to PM if you need to talk.
I think your best option is to give the book back and tell your sister very clearly (write it out and practice in the shower if possible) that the discussion is closed.
And if someone doesn't take the bean dip and asks why you changed the subject I think you can say, "I changed the subject as a polite way of letting you know that I consider this topic to be out of bounds for us."