I am not inexperienced in this area....... I used to wake up each morning thinking about how death wouldn't be so bad and spent nights struggling with legal/medical issues. It's over but it was horrifying and I didn't know if it would ever end.
No one is all good or all bad. Sometimes great good comes from very bad people ( LBJ and Civil rights/Medicare) and sometimes great harm comes from good intentions. C'est le vie.
This sister may be rather sincere in a way that you would not appreciate at present. You have to be physically intimate with a naked old man with bathing and putting on clothes, at a minimum. She may reason that being married to him is the right thing to do, within her religious frame of reference.
You don't have to embrace her emotionally. Marriage for the sake of romance is a rather modern concept and many societies function pretty well with unions based on practical considerations, even today.
I remember writing checks for thousands, week after week, with no end in sight. If I am correct, you are getting a live-in caretaker for about $16 an hour/8hr day - actually much less than that because it is 24/7 - or $2700 per month.
So, if it cannot be grasped emotionally, at least some satisfaction might be gathered by feeling mercenary about it. It sounds to me as if she is accepting a boatload worth of liability in this, whether she realizes it or not. Yes, Medicaid can leave you with nothing, estate-wise.
You may be completely correct that this woman will 'go on to the next' if he should pass away. However, if your grandfather is content or happy at present, within his diminished state ( Best Wishes to Mr Next Up!), then that must be weighed however harsh that may seem to family. Getting old sucks and regrets are many. A friendly face and helping hand can feel very good to those otherwise unaware, even if purchased.
This is not to say that you want to off load him - but you may see this episode differently at a future time - retrospectively.
There is much to be considered....
metatron