sadly i dont think i can give any advice that would solve your situation, but i at least want to tell you that you are definitely NOT overreacting and that none of this is your fault. your brother is being abusive and your parents are basically neglecting your well-being. the way he is acting is NOT your fault and your mother trying to blame you for it is terrible, shes just taking her stress out on you. in my opinion you have all the right to just move away from them, they are treating you really horribly, i get that it's difficult though since they are your family and all the complications with your degree too.
its very kind of you that youve tried to help and support them, but they seem to just brush off your efforts and your feelings and you deserve better. you are not to blame for anything that is happening and therefore it isn't your responsibility to fix any of it either, please remember that. if you feel like youre at your limit and cant try to help them anymore then i hope you will allow yourself to stop and that you wont put pressure on yourself or blame yourself.
and dont feel awful about this, even if your brother is disabled/mentally ill he is still being abusive and you have all the right to talk about it. and like you said yourself your parents just condone it which isnt good or healthy, and they ignore your mental health in the process too which is really bad of them. you are their child too and your feelings deserve just as much attention as his.
