If you have an extended family member who is sympathetic, that is the best. Otherwise, look for somebody who is sympathetic and older with a spare room or something, if there’s some task you can help them with, even better. Older people are likely to be more stable and better understand their own limitations.
If you have an ID you should be able to get a copy of birth cert and social security by applying for them, if your family messes with them they can get fraud charges. You’ll need to file a change of address for disability etc. Try to work things out as possible.
The other thing you should do is see if some anarchist is willing to put an airtag or similar on your brother’s truck to monitor him. This will mean if your brother does end up doing his murder spree he can easily be tied to the murder at the least.
So like Russia, China, North Korea… maybe these aren’t the best role models.
I’d much prefer a rehabilitation system where they tried to make the arsonist a productive member of society and maybe bar them from whatever media was instigating the demand he take action.