Your brother is very lucky to have such a concerned and loving sister. Thank you for writing.
I can’t make a diagnosis on the basis of your letter. I can tell you that I think you are right to be concerned. You were right to talk to your mom. And your mom is right to want to get therapy for you both. You and your brother both deserve the support you need to move forward from trauma. You should not have to live in fear in your home. Your brother seems to be barely holding it together.
With a history of abuse and of being bullied, your brother has plenty to be angry about. Depression can show up as anger and irritability as well as sadness and withdrawal. It may well be that he never did recover from the depression of years ago. It’s also possible that his lack of expression, and his musings about trying drugs, is his way of shutting down feelings that are overwhelming him. It’s possible that his demands that you tell him to “shut up” were the only way he could think of to shake himself out of some kind of dissociation.
I do think sooner is better than later for getting your brother assessed. Your mother may not know that there are therapists who are able to do assessments and start therapy online. Please talk to your mom about consulting with your school counselor and/or your family doctor about your concerns. They may be able to give her names of local counselors who will start therapy via platforms like Zoom. If they are unaware of where to get help, call a local mental health clinic for advice.
Trust yourself. You know your brother well. Your concerns are legitimate. Share this response with your mom so she can get started on getting your brother the help he needs.
I wish you well.