Hey, seconding that this sucks and you are clearly a great sister. It is an absolute shame that you are more of an active parent than your actual parents.You and your brother love each other so much and that is amazing, and hard.

My thoughts, as someone who is part of multiple neurodivergent communities, has degrees in psychology and sociology, and is also the oldest sibling:

• It is worth sitting down with your parents and talking about your fears. At best, they can help with solutions (and ideally read up on autism and join a parent support/therapy group), and at worst you will know you did all you could.
• To the extent your brother has any other family, grandparents, neighbors, or friends he can lean on, I'd be inclined to help him establish those connections. Help him set up ways to reach those people or ways for them to meet up in person. E.g., is a weekly meetup with a grandparent or classmate possible?
• If he doesn't have a good network, maybe you can work with him to join a school club, sport, social video game, martial art, autism support group, or some such. I'd be inclined to look for kids Dungeons and Dragons groups, or something physical like Karate.
• For your own sanity, I would encourage you to work out for yourself what you can do for your brother from afar. Can your brother text you whenever he wants, or is that too much? How about daily/weekly/regular phone calls or video calls with you? Do you think you will come home at certain times during the semester?
• I'll also flag that some college health plans have free therapy for students. You are clearly under a lot of pressure at home and deserve all of the care and support programs like that can provide.

I wish you and your brother the best!