I went through this very same horrible nightmare. Every time I thought it could not possibly get worse, it did. Even though counselors are required to maintain confidence, ours told me on the side that she was going to eventually give me AIDS. After the divorce, her drinking and going missing caused me to just drive over to the ex in-laws and bring my young daughter home. Thankfully my daughter told her she'd rather live with me and the ex gave in. One day, my ex put a gun to her head and ended her life.
One might think that would be cause for celebration, but no. I experienced pain and grief that I did not know existed. That is how love is. All you can do for your Brother is support him and listen. Only time heals.
I learned a few things from my experience, but it took years to sink in. You can never love another person into being what you wish they would be. Tigers never change their stripes. The only choice is to put up with a partner the way they are, or, if you can't, walk.
And finally I learned the hardest lesson - forgiveness. That may sound strange when another is so cruel. But for me it was the final solution to absolve myself of bitterness, pain and grief. And it took years.
I know that does not help your Brother right now. Forgiveness is a steep mountain, but on top, the view is great.