Domestic violence

Hi IMs, I really need some advice. Bit of background info - my dad was physically and emotionally abusive to myself, siblings and mother. My 24 yr old brother has now grown up being an abuser too. He has hit, choked, punched, kicked all his past girlfriends and they have all had the guts to eventually leave him. However, he now has a girlfriend who he has been with for about two years. She has 2 children, 6 and almost 3. My brother has been physically abusive to her on several occasions that I know of. He is nasty in his so called 'discipline' of the 6 yr old boy. He says things like 'dont do that or I'll knock you the f*** out' or 'do you want me to smash you'. He also smacks the boy. They boy can be a little terror at times but aren't all kids! So I feel this behaviour is extreme and excessive. The girlfriend is too scared to leave. My brother is possessive, stalkerish, jealous, demands to know where she is etc.
So now that you have the background info, last night at a family bbq, the 6 year old was walking past my brother and hit him on the head with his hat, not hard and not to hurt. My brothers first reaction was to reach out and punch the boy in the face!!!!! He got him right on the cheek near the eye and a visible bruise and swelling came up. It was a really hard punch. The boy started crying and my brother told him off. I said to my brother 'are you fking serious? That's completely inappropriate, why the hell would you do that?' And he told me to 'shut the f up, you discipline your kid and I will disincline mine, blah blah blah'. I am scared of my brother (very big and intimidating and volatile) so I didn't push the issue. No one else really said anything although they were all horrified. Party broke up soon after. I saw my mum and the girlfriend talking in the hallway and the girlfriend said 'don't let him see us talking, he will go mental in the car.'
So my question is how can I help the girlfriend get out of this disgusting relationship? I know she has to want it for herself but what support or info can I give her? She is a lovely woman and a wonderful mum.
I am so freaking mad at my brother and would love to have words with him but I know it would get me nowhere and I would cop abuse too. He makes me sick, I'm ashamed to call him my brother.