If you want to feel peace inside, Brother Dan, you are going to have let go of the idea that you are responsible in any way for anyone else's "spirituality", including your wife's. That's just another guilt inducing teaching of the WTBTS that she is using on you and you are buying into.
Letting go of that belief also means letting go of the belief that it is your responsibility to get your wife out of the organization. There are a million different beliefs in this world. We either accept them or we don't. Each of us is responsible for what beliefs we accept or reject. Most people accept the ones they were first taught as children. A few people look past that and it usually puts them at odds with their social groups. This is not just a phenomenon unique to JW's.
Remember, YOU don't have to DO anything at this point. Well, maybe take a break because you are tired. If your in-laws believe they have demons, let them get their own hotel room and pay for it themselves. You don't have to do anything.
If your wife wants to go along with them, rant and carry and on and get angry with you, let her. You don't have to do anything. In fact doing nothing is the easiest way to show up their craziness. There they are getting all worked up and emotional and angry, and there you will be, calm, quiet, rational. Nothing to get upset or angry about. Nothing makes a crazy person look more crazy than when they are ranting at a person who is saying and doing nothing. (Note: not going to the meetings is also doing nothing). Try simply not going because you are "tired". Don't argue or try to convince them it is the "right" thing to do. See what happens. You will be amazed at the drama that will unfold before your eyes without you having to do a thing! Then you will come to realize how little control you have over anything but yourself.
Cog