Are your parents in the picture at all? You shouldn't have to be a "mother", you're her sister and just because she doesn't live with you, that doesn't mean that you have to stop being a sister to her. You need some boundaries, decide what you can and can't do for her based on your health and well being. Go to Alanon because those folks will be helpful to you. I have gone and it wasn't as bad as thought it would be, I was kinda scared to go.
Do something nice for yourself right now, to help you calm down and relax, or check out other stories by friends and families affected. I feel better when I read that other people have been thru the same thing and how they coped.
Take care- Lots of love!