I would think the brother would be eligible for Medicaid, probably based on disability.
Also look into these things called Waivers.
My advice is to call a local place that diagnosis people with Autism. Like our major hospital in town has a center for Autism that is staffed with all sorts of professionals. Then ask a social worker there to direct you to resources.
Autism speaks may have some information. I have gotten a lot of packets and information for my son with Autism, but since he will most likely live independently and is still a minor, I didn't really investigate. The fact your BIL needs a legal guardian tells me he is probably low functioning. From what I understand, at least in my state, most of people in that situation are on Medicaid.
Another option is to contact your BIL's psychologist and have him/her make a recommendation for a social worker or someone local who knows the ropes.