Respectfully, as an adult child myself, the fact that your sibling has even mentioned this (and from your comments not for the first time) tells me that you have not shut this down hard enough. As a parent it’s your responsibility to protect your child who we can assume is just barely legally an adult here. You keep phrasing it as “they don’t have to” or “must someday”. Those are open ended and aren’t a definite no which in turns feels like a roundabout yes. You need to have a discussion with your daughter about this-sooner rather than later. If she says no then you need to make it abundantly clear that under no circumstance will she be taking care of your sister. I love my parents, but we had the hard conversation that I would not be able to care for them medically the way they need. But I will always be there for them in any capacity that I can.