How can I get help for my depressed younger sister?

This is going to be long but im desperate. me and my little sister (who is 11) had a hard life growing up in a verbally abusive environment, and sometimes physically abusive (but I think im the only one who got that end of the deal,) anyways, a year ago my mother and step father seperated and my mom abandoned us, completly just vanished, leaving us in an even more so unstable environment with our step father. me, getting older (17) could understand the seperation better than my sister could, although things only got worse at home. the fighting, the verbal abuse. I evenattempted suicide once. after that mess I started to learn how to cope with things, but my sister on the other hand hadnt. so I soon got kicked out of the house just a couple months after my mother moved away (we could no longer contact her) which left my little sister alone with my step dad. It was the best change for me, and although scarred from a lot that went on I was finally living a real life.. on the other hand my sister was, and still is just getting deeper and deeper into this mess and I feel responisble for her in some way cus im the only one who understands what shes going through. I feel like I have abandoned her and she calls me crying all the time saying she needs me and that she wants to run away. I spend as much time with her as possible and try to convince her to live with me but she refuses because she knows he wont let her. two months ago I seen cuts on her wrist one day when I was visiting her.. they havent been there since. but thats not the point! she is crying for help and I keep trying to get people to see how broken hearted she is and that she needs help and everyone just says “its not that bad” im so protective over her and I dont know what to do. im wripped apart by guilt everytime I hear her cry. like she has been forced to start junior high all alone, she started her period all alone, and is so closed off and insecure because she feels like she has no one to talk to. imagine being a twelve year old girl whose parents suddenly split up, her mother and sister out of no where left her, having to go through puberty with no one at home but a step father that does nothing but yell and drink all day. I have seen things she has written on scrap peices of paper about having nobody, feeling lost, and wanting to die, etc. she needs help. my mom is now back in the city (a year later) and I have tried over and over again to make her see but I think shes in denile. nobody is listening and my poor sister needs to get help before she is scarred to the point she cant heal. please help me, im so scared for her, she means the world to me.