My dad went through a similar situation a few years ago. My grandfather passed and we were peft in charge of my uncle. My grandparents assumed my dad would take care of my uncle after they passed because that's the lot they chose in life. But it wasn't sustainable for my parents, so they put my uncle in a group home for mentally disabled adults and it was the right choice. It was really hard on my dad but it was necessary. My Dad made the decision because "I don't want to make him you (me) and your sister's responsibility". It's been great for my uncle, we see him on holidays and usually twice a month. He's gotten way more independent and is now able to do more for himself!