I'm so sorry to hear about that. I'm glad to hear about your sister and surprised. Most JWs would pull out the 'bodily training is beneficial for little' scripture and cancel her gym membership if it meant they could justify something like keeping you two apart. I don't know the exact situation, but maybe that is cause for some hope.
JWs believe they are right, we know they aren't. They're both stubborn positions (though in this case, we are right). I find the best way in dealing with my JW family (and allowing them to deal with me) is the agreement not to discuss religion. If pinned, say yes, you do have some problems with the JWs, but be unspecific. Maybe even warn them that you'd rather not discuss it because it may shake their faith too and you care for them not to do that. (Since my door-knocking days ended, I don't really try to convince people their religions are wrong - except for JWs).
Good luck I hope it turns out OK. But don't antagonize. And beware. Your sister still has at least 3 years under their roof. Make sure she doesn't change her feelings.