Kaos - it is hard and painful to admit that things weren't right, and there are times when it is definitely easier to shelve it. We have lives to lead, after all. But then it rears its head when you least expect it! I have to be vigilant now about my state of mind and care for myself.
I'm okay - spoke to a friend today, went through a few things about it with her (my own real life Stately Home person, bless her) and that felt better. She said that 1) trying to help my mum will involve finding a way of talking to her that she can hear, which is a life's work , and 2) I have to do it in a way that is okay for me. If I find myself getting too het up about it, step back. If I find myself feeling like no-one is mentioning the elephant in the room, maybe mention it, but carefully. We'll see.
I did some reading about codependence which helped a lot. I think my mum shows a lot of signs of this and I might fall into the same trap myself if I'm not careful. Love Wikipedia psychology pages.
About your brother, 12 might be old enough to know better for most kids but that doesn't mean he is evil or anything. Maybe no one taught him some of that important stuff. And regardless of whether he can be held 'responsible' in some sense, what matters is that it will have had an impact on you, and it is okay for you to talk about how it affected you without feeling silly. Whatever the extent of it, whatever his intentions, it is okay to talk about it. But hard, I know.