Yes. Give it until he's at least 18, so an adult in his own right - though still a child in many ways, so the situation may still be hard for him to handle. I wouldn't take it personally if you can't get anywhere with him.
Listen, the last thing I want to do is push my own agenda on you but I think your sister might become more amenable to you in time.
I was the one in my family who bore the brunt of parental crap: the 'scapegoat' in therapy parlance. Like millions of others in my position, I was mega-shocked when I started to realise what my childhood had been like (most people airbrush their history to an extent). In order to start getting my head around it, I did a lot of research on the topic and undertook some psychological work to remember some of the things that happened.
During this period I was very adamant about having my discoveries confirmed by other family members. It made thing uncomfortable between us - their memories weren't the same as mine, of course, though we agreed on certain overall parenting malfunctions, shall we say. The really difficult part about this was they'd say "It wasn't all that bad" but then realise it had been that bad for me. So they felt bad. And then I felt bad, because it's not my job to tell them what to remember about their own childhoods! Gah!
I'm not sure, but I suspect this is why a lot of 'scapegoated' people cut contact with their fellow sufferers. It's a shame, perhaps, because having to see somebody close to you being victimised is traumatic in itself.
Anyway, what I wanted to tell you was this: Once I'd been doing 'proper' therapy for a while I became sure enough, in my own mind, of what happened so I no longer 'needed' the others to confirm it for me. For some reason that gave them confidence - they started to tell me about things they remembered, which was very helpful as I'd blocked nearly everything out; if I added up my childhood memories, they'd come to less than a week!
Result: I got help with my process; bros & sis back on friendly terms; easier discussion of ongoing family problems. All good
It's possible that your sister would respond well to your confirmation & sympathy for her unpleasant childhood. If you don't want to or can't do that for her, you may well find she eases up as she moves along with her recovery. Either way, I hope it works out. You don't get too many sisters to the pound.
Oops, didn't mean to write an essay