Is your goal to maybe coax her into perhaps looking toward finding lodgings of her own, as it's inevitable that your parents won't be around forever? It's not doing her any favors by skirting the issue...and I know you are not doing this...the others are. I just needed more information, as in depth as your explanation was.
As the younger sister of an undiagnosed 46 year-old woman with Aspergers, I need advice. My sister lives with and is still dependent on my ageing parents, one of whom has cancer. The channels of communication have been closed for so long I don't know now to open them. Partly this is due to the fact that I live far away and have a busy family and career of my own now, but it is also due to long entrenched communication difficulties with her and in my family. Now the parents are elderly, we need a plan but there is a family culture of denial, except for my oldest brother. I am awed by his patient dedication and persistence, but even he seems to have given up now. As the youngest sibling, I have often been completely dismissed by my AS sister. All my life I bore the brunt of her bullying and anger. The family is afraid of numerous explosions triggered by raising any issues for discussion, and now one parent is sick they just want peace. But she is lonely and has isolated my parents too.