What to do about alcoholic mother and moving out?

My mother has always drank- going out on weekends and what not. And I did not mind that, as she needs a social life too. For the past few months, the drinking has come to drinking during the week too.. Sometimes 1-2 whole bottles of wine a night. (Usually every other night) She says she doesnt get drunk- Ha yeah right. We had a big fight the other day. My step father was about to leave. She came home from work completly gone one night. During our fight she said she needs alcohol to relax. I told her she has a problem. If my step father leaves, I want out too. I’ve put up with too many of my mothers relationships. And the alcohol is really starting to get to me. Every time I get attached to her “love” they leave. Four years shes been with this guy. I cant do it anymore. I’ll be 17 in a few months. I have a 13 year old brother, and a 3 year old sister. I live in Pennsylvania. I do not have a good relationship with my biolgical father, so living with him is out of the picture. How can I tell my mother I do not want to live with her anymore.. without her exploading? And am I legally able to move out of my mothers house?