My sister is breaking my family apart...?

[Curly lox]

My sister has spent the last few years of her life rebelling. She is now 15, smokes weed, has under age sex and has been excluded from school. She seems to take out all her anger not only on herself (she has self-harmed) but onto my mother, claiming that she was not there for her when she needed her. My mother has always been there for her, so this is completely unfair, and it is tearing my mum apart, as well as her relationship with my dad. She has had councilling etc but she still seems to have so many problems and I dont want her to ruin her future. I know that this is out of my control in so many ways but I cant stand to see my family so broken when we have been so happy in the past. What should I do?
I have tried getting her to talk to me, so have many many other people, and she knows that I am there for her, she is just not willing to spill what her deep problems really are.
She is younger than me and used to look up to me but doesnt any more.
My mother is fully aware of the things my sister gets up to - she isnt in denial but she feels powerless and feels that she cannot stop her, which she physically cant.