What do i do about my mom, i need some help?

My mom and dad got a divorce when i was about ten, after the divorce we lived with my mom and she went out and partied everynight, she began doing drugs and started to become really angry and distant towards my 2 sisters and I. My oldest sister was a senior and she had to stay home every weekend and watch me and my other sister so my mom could go out and do her own thing. My mom was never there for any of us and we had to fend for ourselves growing up. She wouldnt let us talk to our dad, and never let us go visit him. My mom began using drugs and was eventually hospitalized because she overdosed, i was 12 and after that finally got to go live with my dad. Ive had the best life ever with him, and i love my stepmom. My dads a cop and my stepmom is a er doctor so ive had a pretty comfortable lifestyle and its a total different story with my mom. The past couple years shes moved many many times, to trailers, to apartments, to run down houses, and ive had to go back in forth (whenever she felt like being a parent) I'm seventeen now and have gone 2-3 months at a time without talking to her plenty of times, she's missed birthdays, christmas, thanksgiving, everything! Yet she still trys to parent me. She forces me to go visit her, and everytime i see her if i say one wrong thing or even mention drugs she hits me. I cant stand being over there, but im forced to because they have split custody. Ive been in situations where shes put my life in danger. I want to tell someone so bad what she does, so i dont have to go over there, but i dont want her to get in any trouble.