What should my mum do?

my mum is in the most difficult situation. she has 8 boys and 5 girls (i am the eldest) and she really wants to leave her abusive partner (my father) who she has been putting up with for all these years now. she suffered violent physical abuse for many years, now it is verbal abuse but obviously that is still just as bad. i left home when i was 18 because he severly beat me up terribly infront of my mum and she did nothing. she tells me she really wants to leave him now because he has started smacking my younger siblings, one of them being only two. I know the big question is WHY DOESNT SHE JUST LEAVE HIM, but its not that simple. They were seperated in 1994 but over time he has obviously sneaked his way back into her life, bore her all these children and now threatens her that if she goes to get him out all her children will be taken away from her. her self-confidence has been shattered to pieces and is non existent.