The only real way you can deal with a JW that is a member of your family. No matter what you say you cannot convince them that they a working for an enormous tax dodge publishing empire based in NYC.
In return they have to stay away from you once you make it clear you think their beliefs are beyond whacky.
The result - you don't see each other, but you still get to go to all the fun things like birthdays etc without that freak being there. Win!
Bob: Hey, is that your brother in law? Lets go say hi.
Adam: Nah man, he's a JW, we are keeping our distance.
Bob: Sounds like some serious Jehovah's Splitness there bro?
Adam: Damn right, it's awesome.