RM - Maybe you should tell all this to your soon to be niece in law, tell her how you really feel she probably needs to hear it.
I do know JWs are insufferably arrogant and have a superiority complex when it comes to their bible "knowledge" which is very superficial and skewed. They are indoctrinated from infancy that they are in fact God's chosen people after he rejected the natural jews. They're indoctinated to believe they're special and are the only ones who can interpret the bible correctly. They believe they are better than all other religions and believe them to be part of Babylon the great marked for destruction. They look down on "worldly people."
I could go on but I'm sure you will get to know all this in time. It won't be easy for you if you want to maintain a relationship with your nephew once he is married to her. The cult she is associated with is... frankly, quite horrible.
Good luck with that.