Thanks for all the advice here. I've used a couple of things from JWFacts which prove that the house-to-house ministry is not the "be all and end all" of what makes someone a 'true' follower of God. Plus I've showed her that other faiths also evangelise from door-to-door.
I sent all this in an e-mail and since then I've spoken to her by phone but I think she's ignored it. I'd e-mailed her about something else and she replied to that. Whereas she never mentioned the other e-mail I sent.
Perhaps I just need to accept that some people will never see the 'truth' for exactly what it is - however much you put the facts in front of them. When she starts moaning about people at the meetings or the generation/paedophilia issue (which she often does), I feel like saying that I dont want to talk about it anymore as its obvious that she is happy to remain there despite all these nagging doubts.
When she comes out with "where else is there to go and worship that follow true religion?" and "JW's are the only religion that preach door-to-door" then by showing ignorance she is obviously happy with the religion she practices. I am not going to keep wasting my time.