Ok, how about telling them a little about what you are doing now to show them how good they did at raising you (and hints as to what they miss out on) I think I recall you talking about your being a "big sister" and it really seems like you have a great heart and I am sure you have accomplished many things. Mabe let them know that you have some things in common with them besides just a religion?
I just called my mom (inspired by you) and she (of course) didn't answer...but I said "Hi Gramma....We have a crawler now!!" (The baby) and mentioned her two teeth...short and sweet.......just letting her know I may be shunned, but I am not shunning her. Hopefully by showing our parents love will touch something deep in them, and it is a start.
Don't stress it too much, I mean just the fact that you are thinking of them and that you love them says a-lot!