Jez,
Even for all of her professed doubts she is still a JW and they hold a special place for those of us that dislike and attack the org. She was okay with your "other" sins (getting remarried, etc) but once she knew how you felt about the WBTS that's it, that's the line for the most of them that you can't cross.
I have seen the same thing with those df'd. One's df'd for adultry, smoking, whatever, many will reach out to them or on the sly maintain a relationship with them. But get df'd for being an apostate or da yourself and that's it, you are dead as far as they are concerned. I think that is what is going on here with your SIL.
I agree with those that said you have to be yourself and let the chips fall.
BD