My dad used to treat my mum like that, and when i was 9 he just disappeared over christmas, (was working away) and it got to the point where my mum was so upset that she mustered up teh strength to stand up to him, and as sooon as she realised that this wasn't how she was meant to be treated, and started becomming strong, with us children behind her, she was a new woman. It was the best thing ever when he left, we were allowed to do anything that we wanted, having had to be quiet all the time and having to be at his beck and call...
To be honest, you just have to make your mum as strong as she can be... sounds like their relationship isn't really going to last, and splitting up isn't as bad as people make out, for us it was liberation! Don't know if i'm much help, but good luck! I hope that everything works out for you!