Of course you worry about
your brother. You love him, so it is natural, but please remember that all you can do is be there for him and urge him to seek help if he should need it. It sounds like you've really been thru a lot and although we all know how hard it is to deal with depression, not everyone handles it exactly the same or has it to the same degree. Your brother may never get as "low" as you did. In the meantime, try not to dwell on it (I know that's hard to do), as all you'll do is make yourself anxy and sad over something you can't control.
Take care and keep on coming here! Nice folks these!
"If you watch the stars at night,
And you find them shining equally bright,
You might've seen Jesus
And not have known what you saw...
But you would notice a gem
In a five & dime store."
From-"Mary Was An Only Child" by Art Garfunkel