Mate - not sure how old you are, but let me tell you: you're handling this really maturely... I don't want to put anything on your shoulders but have you had a word with your brother about his behaviour and the potential consequences to your family? If no one else is, it may be listened to most coming from you...
Keep it up lad, we're all rooting for you.
As for antidepressants - I can't say I rated them... your Mum may appreciate some CBT (cognitive behavioural therapy) if the doc is willing to prescribe it?
Best
MB