fokily sadly this sounds very similar to a situation that I grew up in 
I was 14 and my brother 11 when my Mum got together with my stepdad. I can't recall how long it was before he moved in but he was manipulative, a pathological liar, strict and a really nasty bully. We weren't bratty at all and in fact was already helping round the house for a few years before he came along as my dad had left a few years before & Mum was working full time so we helped out as it helped her.
We did daily & weekly chores and we'd also have our work 'inspected'. He was a complete bastard and loved being in control. Even when I left for uni and came back home 3 years later, he was exactly the same and I was 21 - but still treated like a naughty child!!
I'm 34 now and in all honesty, feel like my teenage years were robbed from me because of this man. For whatever reason, my Mum has remained with him (even though at one point I admitted to her that I was self-harming because I hated him so much) all this time but now I'm a grown-up adult and don't take any of his crap - he's lost his control over me now so anything he says is water off a duck's back & I couldn't care less.
The point I'd like to make is tread very, very carefully. I recall one time my Aunty (Mum's sister) sticking up for my brother after my Stepdad had a go at him and my Mum has never forgiven her for it and it made the situation worse. We didn't have anyone to 'rescue' us and while I'm close as anything to my Mum, if someone had interjected, I'm not sure we'd still be close...it's a difficult situation and I think you're very brave to want to intervene into the relationship.
My thoughts are with you, do let us know how you get on?