It sounds to me like he has a disability based on what Catherine has said. Yes SSI is for poor people that meet this other criteria. I don’t have to look it up because I know it but forget that others probably don’t. If he doesn’t have a diagnosis he can probably get one.
Oh we all know that people can collect SSI based upon a disability. I think the question is 'What makes you think Catherine's brother is disabled rather than just uninspired by responsibility?'
"Things should be made as simple as possible, but not one bit simpler." ~ Albert Einstein