What can I do? Step Dad being abusive?

My Step Dad is being very abusive, verbally at home. He is VERY aggressive, and screams at people for the smallest things, once he went berserk because half a centimeter of milk was in the bottom of the bottle when I threw it out, and he didn't want it on the floor... Anyways, basically my mum now wants to split up with him, and when she first said it, we wanted him to leave, but he kept going nuts and eventually threatened my mum with physical abuse, before saying he 'wasn't going anywhere'. He calmed down, and we were selling the house, but he's making my little sister, who's 11 years old, miserable because he keeps picking on her. Yesterday my mum asked him politely to stop, and he ends up going berserk and storming off. Now the thing is, we would like to get an injunction, but he earns the money, so if he is gone, then we would pretty much be in trouble financially, at least for the time being until my mum could get a job. I REALLY need help with this, I have been in touch with the NSPCC but my mum says she doesn't want authorities involved yet, and she is scared if we get an injunction he will make it so NEITHER parent has custody of the children just to 'get even' with her. What can I do? I live in London, in the uk, if that is any help. Thank you so much!