To be clear, I don't blame you, Cow, if you don't or can't help your niece. I don't particularly blame my parents for anything they did or didn't do, or anyone else -- sometimes I wonder how some things could have happened the way they did, but I know everyone acts out their own part, and some parts aren't heroic in every scene. Or even, ever. I'm no hero.
I did spend about a decade of my life working a lot of long hours to alleviate a little bit the misery of a lot of seriously messed up, addicted, alcoholic, abused, violent, and depressed adolescents. This was during my active alcoholism. I know I'm not alone in that. I even took some of my kids to meetings. Apparently I was pretty good at the job, in my way. So it can be done.
AO, I like your attitude. Anger can be good.