Maybe you could try to contact your sister beforehand and test the water...ask if for your parent's sake, could everyone just get together as family without reference to anything else (meaning you congo status).
State that you do not want anyone to make mom or dad feel uncomfortable, and this, after all, is necessary family business. If she is willing, make plans from there.
Perhaps it would be better to just keep it at family, since sis would have to go against her principles if any other JWs came. Maybe make a CD of family pictures to play in the background,including some from their wedding, or a scrapbook of remembrances.
I would remind sis that the day is not about you or her...not a time for anyone to get on a pedestal or a "high horse."
Hope it is a lovely day for your parents.