Good advices!
All drugs can mess up the balance between feeling good and happy and feeling like shit and unhappy. It may be that your brother is using it chronically to escape something that makes him unhappy or maybe he used something strong that really messed him up.
I want to give you some advice because I have a caring and loving sister that wanted to help me in times of depression, but I simply locked her out, in retrospect I think because I wanted to avoid having to face emotions of guilt, fear, despair etc.. I'd rather go sit in my room a smoke weed, which of course didn't help me, it only made things more difficult.
The best thing a sister can do is spend time together without giving him the feeling that he needs to talk about his issues (take a walk, make music, watch a movie, etc.). Him doing nothing and locking himself up in his room chronically is digging a deeper hole for himself; it won't help him at all, been there, done that. Tell him that you won't force him to talk but that you're there for him if he needs it or if he simply needs a hug. Hugs are always great, because you send a reassuring, loving and comforting message without having to use words. Every time I see my sister nowadays we always give each other a good long hug and I can tell you that it really feels good for both me and her.
For the rest I advise him to see a professional to help him further.
Take care!
I just hit the post button as you posted your last reply. Good to see you can reach him.