What should I do?

Ok, So im having really big problems at home. My mum had a kindney transplant and before that I use to clean and cook, babysit my nephew and still went to school. Since she had the transplant all we have done is fought, about 5 months ago I left home and stayed at a mates, then mum and dad came after me and talked me into coming home by saying we will change and stupid me belived it… When I came home it changed for a while and then it went back to normal, the abuse and the just everything!! I am only 14 but I have grown up so much faster then my mates. I am more like a mum to my nephew then an aunty, I look after him and all the things my parents should do. ( he lives with us not his mum ) My mum is cheating on my dad and I know it because I saw the txts on her phone from her ex boyfriend. My parents were happyly married for 9 years but thats all changed since the transplant. My dad has always hated me and I dont like to be around him with out someone eles because im scared of what he will do. In feb my mum is going to away for about week and im thinking about running away the first day she is gone. The only problem is that I dont wont to leave my nephew behind because I recon that my dad will hurt him with his drunk rages. So I was thinking bout taking my nephew with me. I have a safe place to go to ( really ) and I would feel safer there then with my dad. My nephew is 6 and im 14 and I know that will be illegal and that my dad, mum and police will be after me if I do run but what eles am I surpose to do. I have $529 saved and another $149 coming in the bank tomorrow. What do I do?? the only person im worried about hurting is my mum because 2 of her other children left when they were 15… What to do?