What a horrible story. I am so sorry you are being blamed for your sister in law's illness. She really does need to see a doctor and I hope they finally get her the help she needs. As far as JW's being superstitious, I recall from my childhood, playing records backwards to see if they had messages from the devil. I lost so many of my favorite records because my mother heard something she did not like. I recall renting movies at the video store and then watching them and something would be "satanic" and my mother then wrapping the video in watchtowers and dropping it in the drop box at the video store immediately. When my husband and I married, and my family converted him for the one year he was a Witness...they convinced him that his Magic the Gathering cards were evil and we had a bonfire in the backyard and burned them and his Dungeons and Dragons manuals. I've since replaced them for him.
And the saddest story ever was of a young brother who was my age and diagnosed schizophrenic. The elders were convinced that he was possessed. Even though the doctors told him the voices he heard was just a symptom of his illness he thought it was demons talking to him. I was one of the few kids in the hall he could talk to that would not freak out on him over it. He would see things all the time and hear things and had quite a few scary stories. Sadly he ended up killing himself a few years ago. I guess constantly being told you are demonically possessed would get to you.