T.S.
OMG your niece sounds just like me!
When I was 14 I was totally in charge of my younger siblings, while my mother ran around partying and occasionally "working."
I was so fortunate that my aunt and uncle took me in. They couldn't take my siblings, but they offered to take me so I could have a normal high school experience, and a normal life. I was able to do sports, cheer, drama, yearbook, have friends, go to games and dances, work part time jobs, basically everything I would not have have been able to do if I stayed with my mom (in addition to me being the live in babysitter we moved 3 or 4 times a year.)
I can't really advise you in your specific situation but in my case I think it literally saved my life!