Your sister is very lucky to have you for a sister. It is obvious that you love her so very much. You are doing something very important by posting in this forum. You are taking a step to help you.
The feelings of helplessness and worry are so strong. The concerns we have about our alcoholic loved ones (in my case the A is my son) take over our lives. The alcoholic is numbed while those who love them deal with the fallout of their problem stone cold sober. It's a terrible terrible feeling. It is particularly sad when there are children involved.
You said some very powerful things in your post. You feel helpless. That's because you are helpless to help her but you are not helpless to help you. And that's a great start.
I remember when my alcoholic son was at his worst. My worst nightmares were that he would get a DUI (he did). His girlfriend would move out and take their son with her (she did). He would die (he didn't).
Sometimes the things that frighten us the most for them are the things that finally get through to them. We love them so much we are always trying to control (and lessen) the consequences of their drinking. Between being numbed with alcohol and having loved ones protecting them from the consequences of their drinking, they are able to continue. That doesn't mean that their drinking is our fault (we didn't Cause it, we can't Cure it, and we can't Control it = the three c's). But when a family revolves around the alcoholic, it isn't healthy.....for any of them.
I was like you (but dealing with my son instead of a sibling) in that I just couldn't give up on him. I felt like I had to do something. It was just too painful to watch him spiraling downward. I couldn't bear to lose him. Eventually I realized that I had to do something to stop my pain. The pain of his self destructive behaviors were killing me. That was the starting point of MY recovery........and luckily, the start of his too.
Take care of yourself.
gentle hugs to you as you begin your journey