@pleonasm - You're assuming that the family in question has the ability to rearrange themselves for this, or has the money to provide any kind of care at all.
I have heard of so many people, often women, who live desperate lives because they are the sole carer for someone with severe, profound mental retardation. There isn't always options to make sure everyone is happy and stress free, and care is expensive, whether it's a group house or someone visiting the home.
When there's no one else around to help, it really does become a more than full time job. Which basically means that the person with the responsibility never gets to live their own life, because if they aren't there, no one is.
It's a terrible thing and something that we should not allow to happen in the modern world.