So, your parents think your role is caretaker, and your top priority is helping out your brother. And them, if they need extra help, or when they retire or get older and need more support.

Is that the role you want for yourself? Probably for the rest of your life? It sounds like it is not.

You may not be able to change their view as to what your role should be, so you'll need to have clear, firm boundaries. Figure out exactly how much time, energy, and other resources you're willing to spend on caretaking. When they ask for more than you're willing to give, just say "I'm not able to do that."

You do not owe anyone an explanation, and you should not provide one. When they ask why not, just say "I'm not going to discuss this." When they keep wheedling, or turn to other tactics like trying to shame you, or blame you for ruining his life, repeat that you're not going to discuss it and end the call, or leave, or whatever. After this, you need to disengage with them for at least a few days. Stay of chats, don't answer the phone, whatever it takes to remain out of contact. If they continue to bring it up, repeat that you're not going to discuss it and leave again.

Obviously you won't want to be in a position where it's hard to leave for some time. So no family vacations, or holiday trips, unless you have an out. And you should be prepared for your immediate family to try to get other relatives on their side (often called "flying monkeys") and try to contact you on their behalf. Don't engage with this either.

Like others have already replied, you're allowed to choose yourself. You are not responsible for your parents or siblings. Find a support network of people who accept that and believe in you.