An incredible burden. My sister is in a similar boat. Has to live in a group home, but hard to know what'll happen when my parents pass who are her default guardians. I know me and my brothers will step in. She's age 38 now, and she lived with them until about age 32... imagine living with someone who wets the bed every night, up at 4am, can't go to restaurants or trips, has a consumption issue where she'll drink a bottle of mouth wash or swallow batteries because, who knows.
I was very nervous when we were having our 3rd. Growing up with a sibling with special needs, I honestly don't think I could do it again, it's beyond exhausting. If I ever won a billion dollars, my charity of choice would be to provide resources and relief for dedicated parents carrying that extra special burden for decades.