Go get em nic!
The good news is that your nephew is not completely encased in the bubble like you and I were. Jehovah this and Satan that was all we knew and that is what makes the dubs powerful. I know you said its not your kid but the mental capacity of your average 10 year old (especially with the encouragement of his uncle) is enough to keep this little shaver out of the cult. Help him reason and not to mention inform his parents and you will be good to go!
I also read the post about your brother and I feel a distinct kinship with you because of that story although mine is still in.
Good luck,
pbrow