Pogostick, it sounds like you're on your way. Congratulations on setting up Team You -- you will need them while you navigate this.
My only advice, and this comes from experience, is to really, really educate yourself on what you're dealing with. That includes taking advantage of support groups like Al-Anon or these forums, so you can get a real-world picture of how boundaries work and what "detachment" looks like. Since your fiance's mom died of alcoholism and your fiance's brother is experiencing (what looks like full-blown) alcoholism, this is going to be a facet of your life as long as you are in this relationship.
Like everyone else has said, there's nothing in particular you can say or do to help him see the light. But I did get one piece of advice that helped me look at the alcoholic behavior in a new way. Someone said that the alcoholism will fill up the space you give it. Your goal as an enabler and a witness and a family member is to stop giving it space. That might mean the brother has to move out. That means you might stop cleaning up his messes and paying his bills. But ultimately it means that you let him live with the consequences of his drinking, and let him have the dignity of making his own choices, good or bad.
The good news is that you get to decide how much space this takes up in your life, too. That's what good boundaries are about. And if experience serves me at all, my thought is that once you change the way you've been dealing with this, you're going to be much less tolerant of the chaos and drama. You don't need anyone else's permission to deal with this in your life, not even your fiance's. You get to decide how you live.