you're the right person to do this.
if your niece is unable to communicate to the doctors whether or not something should or shouldn't be done, you can question the doctors as to why they want to do what they want to do and if it will make things better or worse. then make the call. it's as simple as that.
no doctor is going to make you choose between a right or wrong answer, so all you have to do is ask the questions, understand what may or may not happen and let the doctors know.
any friends or relatives that need to know what is happening or what happened, just tell them the basics, don't tell them it was you that made decision(s). just "a" happened so the doctors did "b" to help but it didn't work, or "a" happened and there wasn't anything they could do. NOT: "a" happened and I told the doctors not to do "b" then people start talking... well you know how people get.
when I've faced things like this I like to think about that scene from the godfather where johnny fontane is asking Don Vito for help, crying "what do I do, what do I do"... Godfather "You can ACT LIKE A MAN!" not saying you're crying or anything, I just like that quote when I'm faced with important tasks and feeling uneasy... somehow that makes me cowboy up.
good luck you got this