Thanks for all your replies. I actually just phoned this sister, and it went ok. She was very friendly, and sounded so glad to hear from me again. We spoke for about 15 minutes, and she asked if I'd mind her calling me again, though we didn't arrange to meet this time.
She thought I sounded happy, and asked about Trev. The one "awkward" moment was when she asked if we have any plans to marry, and I said we hadn't given it any thought. I guess the jws, or most of them, see themselves as occupying the moral high ground, and the idea of people living together is a little hard for them to take. I thought the reference in her letter to our sex life was a little off, but dedpoet laughed about it. I didn't say anything about it to her though, I am just happy, and a little taken aback, that she would still like me as a friend even though I'm da'd now. She is after all taking a chance in contacting me at all, knowing what her elders are like.
There was no hint of preaching at all, and no attempt to invite me back into the fold. She seemed to be genuinely glad to know that I am doing well now. I told her she can call me any evening, and I didn't mention it but I would probably agree to meet with her, discreetly for her sake, in due course. Dedpoet suggested we might even lure her out of the wts, but unless things have changed she is a very loyal sister, so yhay might be wishful thinking on his part. I will see how it goes, and keep you posted if and when we talk again.