There has been so much good advise - and depending on your family, your personality, beliefs, etc. in amonst the range of suggestions I'm sure one will hit home.
I faced this with my family, and like you, I have one sister "out". At first the pressure was on to attend meetings, "come back", and we finally reponded that the only reason we would go back now would be to have a family relationship. It would not be motivated by anything else, which satisfied my parents and they have quit the pressure.
I believe wholeheartedly in unconditional love. It is not something taught by JW's. If bringing out issues on thier faith will cause more disruption or hard-feelings, I would be inclined to avoid it. I would acknowledge their feelings and as was suggested by Maverick, treat them normal. YOU DON"T HAVE TO ADDRESS THEIR EMAIL. I'd ignore it. I'd talk about my life. It worked for me. I've had some distrubing letters, but I write back that I was glad to hear from them and I volunteered for the Food Drive. I've mentioned positive things I do in the "world" that we could not do as witnesses. They've softened over the year. I do not hide my beliefs (or lack of them), but I chose not to confront emotionally charged issues. As Amazing brought out, the purpose of these letters is for control, manipulation, etc. You don't have to play their game.
My 2 cents. and BIG HUGS - its tough and there are no easy answers!