Welcome, ordinaryworld. Sorry for the situation that brought you here, but glad you found us.
Read the "stickies" at the top of our home page - lots of good information there about alcoholism.
I can't answer your question - my sister is not an alcoholic and she does the same thing! We are in a hardly-speaking mode right now; I haven't had an actual conversation with her since she moved out of the state I live in last June.
But I can say that "high functioning" is a phase for alcoholics... and erratic behavior can be a sign of a alcoholism, but there are many others, too. Have you ever tried to talk with her about her drinking?