Thanks so much to you all for your advice and support.
I rang her last night and it went great. She said she didn't have any hard feelings, alghouth she didn't understand why I had taken such a hard line against her. Apparently my elder father and mother have been contacting her about 3 times a week (they never would tell me that). So she thought it was just ME. I told her that even her parents weren't "allowed" to have this contact and that dad could be removed or even DF for it.
She was really glad I called. We've agreed to stay in touch and visit each other and get the kids together as soon as possible.
I had to get her phone number from my parents. My mother first assumed that I was dying (why else would I be making contact). When I assured her I wasn't - she then was worried I was going to be rude to my sister and scare her from ever coming back to "the truth". I told my sister all this. She said that my mom has been hounding her to find out why I was contacting her. I told my sister to have some fun with it and tell mom that I berated her for an hour. She thought that would be great fun. She'll let mom off the hook before too long though. (We were always teaming up playing jokes when we were young).
Anyway - I've gained a sister, brother in law, and two neices. I haven't talked to my parents yet, but it looks like I can still have a relationship with them even if I'm out.