Being a JW in the past is not something you should be ashamed of, as if you were a criminal or something. Once you are in a relationship where the issue of family comes up and you feel you can trust them, you can talk about it. Just be honest, as honesty is a crucial cornerstone to any successful relationship.
You can say something like "my parents/family are JWs and I decided it wasn't for me but their cult doesn't accept freedom of thought and choice, so they shun me. That's what their religion tells them to do, as crazy as that sounds. So we don't have a relationship because of that. Not my choice, as I would associate with them no problem but they are told they are not allowed to have anything to do with me, to pressure me, through emotional blackmail, to go back."
She'll probably be shocked and disgusted, as everyone I've ever told that to has been. But they'll be understanding and they'll not hold that against you.
The best way to give that cult the middle finger is to live a full, happy life on your own terms.