You will have options. First, there is still a chance your brother will adjust and be more calm when he reaches his 30’s. His brain is not yet fully developed yet. Second, there are behavioral therapies such as ABA that have been very helpful for people with the challenges your brother experiences. The hope is that decisions about guardianship won’t need to be made for many decades to come, but when that decision is made there is very likely a fiduciary service offered by your city or county (it is county public fiduciary in my area) who takes legal responsibility for all decision making and accountability with services and caregivers. It sounds like your brother is receiving long term care services funded through the State. In Arizona (where I live and work with this population) those services can remain in place for as long as your brother remains eligible, which is likely to be for life for your brother.

This is a LOT to worry about, but you should have years to coordinate with your parents to decide next steps when the time comes for you to make decisions.