Ok, so you know what it's like to be left (as a child) to care for an adult that isn't being a parent. That is enough to tell me that you have the strength to do this. You may not "want" to do this, but that doesn't change that you "can".
Given her propensity for alcohol. I think I would limit what I told her. You don't know what she'll do or say esp when she's been drinking. You don't have to "keep" it from her, but you can choose what to disclose. Whatever you tell her, I'd keep it simple and let her know in no uncertain terms that she is not to interfere. If she does esp if it is behind drinking, there will be consequneces.
I know how hard this is for you. We all do. I've been living on the razor blade for more than a year (post bomb). I work part-time at a minimum wage job b/c I've been unable to find anything decent within a reasonable distance from home. H (so far) is still supporting us financially and I never know what's going to happen there. I just keep pushing forward to take care of my D's and myself, get a job and update my skills. That's all I can do. Whatever is thrown my way, I just have to deal with. I can and so can you.