I am so awfully sorry about your brother- drinking truly does ruin the lives of the whole family, not just the individual.
My Mum died last year from 'chronic alcoholism'- she was 51 years old. My family and I had tried everything we could think of to get her to stop drinking.
You never think it will happen to the people you love. When I heard the news that she'd died, I went to her flat and found so many empty bottles of whiskey (that she had clearly been drinking neat)- so I really do understand how you must have felt upon seeing the same kind of thing at your brother's place.
It is a massive cliche to say this, but you really can't help people who won't help themselves. If you are hammering on their door, screaming at them to get help, to stop doing this and promising them that you will do everything/anything to help them, if they're not ready to help themselves, there's really nothing that we can do.
As someone who is slightly further along 'the line of grief' than yourself, I can tell you that it doesn't get better- you will never get the answers you're asking yourself and you will never stop feeling guilty because you think you could have done more. However, I will say that you start learning to live your life for the one you've lost- every time you try a new activity or go to a new place or laugh or just generally have a really good day, you think of them and you may be sad for a moment because they weren't there to enjoy that time with you, but you start to realise that they would be happy that you got to enjoy and do those things.
Life slips back into some sort of 'normality'- they're always there, but it goes more towards the back of your mind rather than constantly being at the forefront from the minute you wake up, like it is in the beginning.
I really, really wish the best for you *sending hugs*