I'm so sorry you're going through all this.
Allow yourself some grief for your sister, that will help your sobriety. If possible, seek some grief counseling or therapy. Self care is your focus right now. Your niece has a safe harbor at the moment, and you want to care for her. You need to care for yourself and have a solid foundation before you can care for others.
I'm not a big AA fan for myself, but I think it's a really good idea for you if you find that it works to keep you sober. The community will understand what you're going through, and when it does go to child protective services (or whatever it's called in the UK) the fact that you are going to meetings speaks volumes. Not sure what the system is in the UK, but in the US you have the secretary sign for you that you attended the meetings, as a lot of people are court ordered or in professional rehab programs that require 12 Step attendance. Document every meeting you go to.
And of course use the program to help you stop drinking.
If you find that AA isn't working for you, there are other options, but if it were me I'd just keep that in my back pocket for now. And we're all here for you.