Hi there and welcome. Well, first, I must tell you how sorry I am to hear that your childhood was as volatile as this. An alcoholic mother that loses her children is no small thing. I can only imagine how that felt to you as a child and that you probably to this day have emotions about it.
I agree with anniebrooke that often what happens is that people take on certain 'roles'. It is not your job to fix problems other family members have with one another. I think that an alcoholic step mother that hurt you as a child would definitely be someone that it is appropriate to cut all contact with. some people have proven to be toxic in your life and when that happens, disassociating with them is appropriate action and what one SHOULD do for peace in their own life.
I am not sure of the reasoning for her actions with her sister. You may not be either.
All I can say is that when someone has lived with an alcoholic parent, it does deep emotional damage. Your older step sister, the one you feel cuts people out and yourself have hopefully done counseling and are still doing it. What happens to us as children can stick with us and form adult patterns.
An example may be that when you were a child, you wanted everything 'better' no mater what. Now as an adult, you want situations like your step sisters and their choices to be better. In truth, maybe this is how it should be or maybe the younger sister is making mistakes based on her own emotional state. But it is her journey. You control only your journey.
As Anniebrooke said, be kind to both sisters and keep your relationships positive but let them work on their own and you try your best to stay completely out of it. good luck
I think you don't need to, and can't, "fix this rift," even if you think you know why she is the way she is and what she should do to be happier. Guess what, she is doing what she can to be happier. Right now, she is at the limit of what she can do. And there might be more to the story of the two sisters' relationship that you do not know and she doesn't want to tell you.
Coming from a family with alcoholism in it myself, and believe me it is with empathy that I suggest this, it sounds like maybe the fixer role was one you took on as the child of an alcoholic when growing up, to create stability or comfort in world that was chaotic. I'm sorry, but even with such a strong imperative, you can't fix other people nor do you have the right to insist. Your stepsister has the problem and the right to live with it as it is. (Even if she were closer to you, such as your child or spouse, you are still not in charge of fixing their lives; they are.)
Your stepsister is coping in such a way as she can. People will open emotional cans of worms only when they are ready and strong enough for the sadness or loss that they find. Later in life they might heal a bit and be able to do more.
All you can do is love your stepsister and let go of your desire that things be different for her. Just be there for who she is right now and what her state is right now.