If I were you, I would focus my resources on helping the kids. They are trapped in the situation, so by being a stable presence in their lives, and by modeling what healthy relationships should be like, you will be doing a wonderful thing.
I was one of your brother's kids. I can't tell you how much it meant to have a safe, supportive place to go. For me, it was a friend's house. I thought I was great at hiding my home situation but, in hindsight, I know they knew. They never said anything about what they knew/suspected but always made me feel welcomed and included. They even brought me to their familly get togethers and always had a little something for me to open on Christmas/Easter gatherings (like little wind up toys, nothing big but it made me feel welcomed and liked).