A while ago, I was in a similar situation, just without the drugs. I was going through life not seeing a reason why I should continue living other than the fact that I was breathing, and that gradually became not enough of a reason. I too didn't care whether I lived or died the next day and would have welcomed it. I did reckless things not caring whether I might be injured. Why worry about consequences when you're looking for death? And demeaning things are only demeaning if you still care. There was no point to living if I can't see a purpose for it. I saw too many people around me going through life with a purpose and direction that I didn't understand or feel. I needed more than living just because I was breathing and because society expects me to go to school, get a job, have a family, raise 2.3 kids and a dog, pay my taxes and die like a good little human being.
The drugs and the drinking don't help, but You can't help the body if the spirit no longer cares. The spirit needs something to hold on to, a goal, a dream, something that says this is why i want to keep living, to strive for. Something I didn't have. It took a half-hearted attempt to kill myself and time spent in a psychiatric facility for me to work out for myself what I wanted out of this life. I want to fall in love and to raise a family, and that is the only reason why I still breath. I may not realize my dreams, but I want to live to at least try.
Your sister is more fortunate in that she has goals and dreams. She wants to escape reality and run away because she can't see a way to resolve what she wants to be and what her current reality is. I wanted to escape because i saw no point to continuing reality. You have to help her realize and identify for herself whats she wants most out of this life. Not "what do you want to be when you grow up", but what do you want to accomplish in this life.
Sometimes you have to have the courage to do whatever it takes to help, if she doesn't have the courage for herself. Feelings can be mended, a lost life and a lifetime of regret can't.
I hope my story helps you see a way to get through to her.
