I'm going to tell my wife that I no longer want to help take care of her father.
Do you not want to help because you've been paying for everything?
Yes, I don't feel like helping anymore. It makes me feel as if I'm supporting him as an adult.
Since you are married, the well-being of your wife's family should concern you since you are now related and you should give your best effort at helping them if they need it during hard times. It would not be fair for your wife to shoulder that entire responsibility as spouses are to help out each other when it comes to family matters. You should continue to help out your father-in-law or otherwise your marriage will become strained and your wife may want to file for divorce.
That's easy for you to say, you're not paying the bills! I'm in the right here!
Have you thought about it from your father-in-law's perspective? He probably would rather not have to rely on you to help care for him either. I think you need to keep helping as best you can and you'll see, things will work out in the end!