An adult nephew or niece taking full responsibility for mentally incompetent aunt or uncle, what do you all think about this?

Meaning that when their parents die, they went with a sibling, then when the sibling and spouse of sibling died, they went with their adult niece or nephew.

I think this is very nice and admirable that they're willing take on responsibility but this would most likely dampen the life of the adult niece of nephew as the mentally retarded adult would also be old and require alot of care which could keep the niece or nephew from going out with friends and stuff like that. Your thoughts please.