acm---I agree that this is tough, and, I am sure, very painful!!! You must feel a tremendous sense of betrayal. I know that I sure would.
Your feelings are valid (all feelings are). It is a matter of what you do with them---which is what you are asking--right? I would say, first of all, don't feel guilty for your feelings. Allow yourself to feel them. And, then, go from there.
The boyfriend is history---good. No more problem there.
Perhaps, in time you can forgive your sister---understanding that she has the illness of alcoholism. That doesn't make it o.k., but it is the reason that it happened--probably.
In any case, it is important that you don't let this "eat you up". Find a way to get past it, eventually. Life is too short--I'm sure you know this...LOL.
I would not trust my sister around any of my male partners, though. You know what her limitations are (now).
You can still love your troubled sister, and you can still have her in your life--with the proper boundaries, of course.
This is just my take on the situation...... how I would look at it if I were faced with this very upsetting and painful circumstance.
dandylion