Maybe the Uncle should fien interest by attending studies with her ,and bringing up thoughtful questions she may not be mature enough to come up with yet. Attend meetings with her and make observations she may not be seeing . Since her parents don't seem as plugged in she is lucky to have an Uncle that cares .
I could not say it better than Nugget has already stated .....teach her some critical thinking skills through conversations . Avoid attacking the JW religion ,but bring up thinking type questions . Help her to think ahead (something most young teens don't do well )