All you can do is be there for them when they realize they've been sucked into a cult. The bible and church isn't a problem, so maybe lean them towards other churches if a faith community is important to them, and maybe they'll see the problems with JW. Cults target people who are lonely or going through difficult life changes, so building your closeness with your parents may make them realize they have all the support they need unconditionally, rather than the conditional love the Watchtower Organisation gives them.