If they can't accept you when you're not officially a Jehovah's Witness—even if you're not a bad or immoral person—why should they matter to you? "Because blood"? Blood relations are random. They only have meaning when there's acceptance and love. Otherwise, they're meaningless. (Sorry if it seems a bit harsh to someone; I was invited to share my viewpoint.)
I'd have to agree with just fine. Move along. Shun them back. Write them a letter explaining why you feel obliged to shun them.
When—if—you'll find the right person, you'll see that she'll accept you no matter what. People are better than you may (I'm not totally sure whether you do) think. When you tell her about your past, she'll understand. She won't hate you. She'll hate your Jehovah's Witness family because they won't accept you. There's no reason to hold back who you once were. Embrace it. Live with it. Move along.
Edit: Just saw what LifesNotOver said. I'd have to agree that it's better to tell her about your past matter-of-factly and when you know her a bit and the time is right.