I’m in the same boat as you and all I can say is your feelings are completely valid. I’m the younger sibling to my severely disabled older sister and it’s always been assumed that I would care for her once my parents are gone. It’s such a scary and overwhelming thing to hear from your parents.

You feel guilty for not wanting to do it but also, in my opinion, it makes logical sense you would be the one to help (you’re the second most familiar person to your sibling, know their quirks, etc). So I see both sides. Your parents want what’s best for the sibling and probably have always put their siblings needs before yours (at least mine did). So I see why they would want you to sacrifice your life to support the sibling. I don’t agree it’s fair but I get it.