I'd say you're just having a moment. Bad things have this habit of waiting until the wrong moment, then gathering together like a virus and sweeping you down when you least expect it. Concentrate on the positives for the time being.
Maybe its all the bad things building up and sometimes you just have to have a good cry to let it all out, but i'd say try and be positive, try to always remind yourself of the good things, and try and be there for your brother, he seems to be going through bad-times too.