My gut is with Cimarron, and I stand by Will, Ellie, Levite and Mixedmedia in their responses. So help me I do
You are feeling for your brother, as indeed you cannot not. Please add to your understandable empathy the following fact: There are men who have, as children, been abused - raped, and who, like in your family, have had it kept in the dark, yet have grown up to be adults who have refrained from echoing it onto THEIR children.
He chose to carry it differently from them.
In such families as yours, there are those who witnessed yet were dumb at the time, who, later in life, have, themselves acted out abusive behaviours. You have not done so. You have chosen to carry it differently from them. There is no guilt in you, and the virtue you carry is all yours.
In your topic title you wrote: "What the hell do I do" There will be much you can and will do ... and much that you cannot do and in those moments you may know fully how this, unlike the time of your childhood, is in the light, that there is much you are no longer carrying alone.
Bloody hell
Take care.