I also agree with landthatilove, if you do not report the issue to cps, there's not much else you can do. A person that makes excuses for their children that way, are more harmful than anything because they don't put a stop to it and let it get completely out of hand. As much as your mother "loves" your sister, she's hurting her even more and therefore is not a suitable parent for her. Reporting her makes the difference between your sister being in school, graduating, furthering her career, living her life age appropriately and ultimately enjoying it making something of herself OR being another statistic of teen pregnancy, drinking, getting addicted to drugs, not being able to hold a job, making the same mistakes with her future children, ending up a miserable mess, possibly even dead at a young age. Kids need rules, boundaries, and discipline to shape them into responsible independent self-sufficient beings. Something, this young girl will not be if she keeps going down this road, unfortunate but true. I'm sure you want someone to tell you that it'll be ok and she's listen and turn around. But, honestly, if she hasn't listened yet it's extremely doubtful she will any time soon. Also, if the mom hasn't done anything till now, she has no control and can't possibly do anything and doesn't seem as though she'll get any sense knocked into herself either. Not making the HARDEST which is also the BEST decision, is selfish, cause this young girl won't stand a chance.
Also, she definitely needs psychological help because there are obviously some underlying issues here that she hasn't dealt with which drive her violent, disruptive, disrespectful, and self-destructive behavior.