Well, OP I can relate to you to an extent. I have a younger cousin who does have the same problem as your sister. If I did have to deal with him daily... well... I'd probably make this topic (oh the shame)
But you bring up a good point, his sister will have to take care of him right now and essentially his parents go through everything just to take care of him right now, god damn it I would NEVER want that responsibility but while I do think it's wrong that they are forced to be taken care of by siblings once their parents die, saying they should be segregated is too far.
However, if you made up a topic about who should take care of the mentally disabled, you'd probably receive far less flames.
Trust me, we don't do that. Society is very judgmental, we praise it when they do anything minor no matter what and we tell ourselves that no matter what negative action they do they aren't at fault (I could reference how a while ago, a mentally disabled went up to kill his caretaker because he saw it on TV). Not saying that morally it's wrong, but that's how it is.