My little sister is taking drugs!?

Well my sister is 16 years old,and we recently found out she smokes weed, I told my mum straight away ( out of concern not spite) it all blew over she lied about it but mum does not believe her, but there is not much she can do. But I recently found out she may also have snorted coke, and taken e's, Again I told my mum, I also told her that I didn't no if it was true, but to keep an eye on her. A couple of nights before I herd my sis on the phone saying about her boy friend being pilled up and smoking weed, so I ANGRILY asked her if is she was still smoking weed she said " not really " I said not really or no she answered not really, after I told my mum about the coke and e's my mum called her in the room and we sat down and talked, my sister got all aggressive and nasty and denied it all, so I repeated the hole conversation and she called me a lire and said I made the whole thing up, this has all happened since she got with her new boy friend, so my mum asked for his full name because she wanted to do a police check on him ( my cousin is a police man so he can check up on this) she just wanted to see if he had a history of drugs, just to find out the truth as she is obv worried about my 16 yr old sis,when my mum asked for his full name my sister started screaming and stood up to him my mum, with this I lost my temper, jumped up out of my seat and pinned her against the back wall, throwing her across the room, I didn't mean to and I tried to apologize but she wouldn't talk to me, to make matters worse this really really upset my mum, I am pregnant and she would rather have my ill sis hit her than me get stressed out, but im not going to let that happen, im just sick of her attitude and the way she treats my mum, my mum can't through her out because she's got no where elts to go and she's scared if she does she will do heaver drugs, I don't no what to do, my dad isn't around and my mum is really ill, my step dad gives her everything so we don't understand what's going on with her. I just want to help, iv thort about moving back home to help my mum out but I can't really do that with a baby on the way, please help? Please don't tell me its not my problem because it is, my mum is really ill and can't be doing this on her own, my step dad can't do a lot because she hates him and wont give him the time of day, even though he is a grate guy, I just feel so sorry for them, my step dad also gets really stressed because he can't help my mum?.....any Advice? Its getting so bad my older sister is thinking about quitting her nursing job in leads and moving back home to help out, I really don't want her to this,she has her life job and home in Leeds its not fair on her, she comes home as often as she can to help!