The specific form of "mystical" power which Justine has is precognition. And she's always had it. That's the root of her depression: She knows how things are doomed to run their course...always.

That's why she agrees to get married and throws the big gathering, even though she knows the world is going to end and she doesn't really love her husband. She wants to get everyone who is important to her (her family and business associates) together in one place, so she can finally confront them one final time.

She believes everything is meaningless, and that there will be nothing after the End of The World. So she decides to be cruel and ruthless in addressing existence, and telling people exactly what she thinks of them.

But the point of the entire film is that, when it comes down to the Big Crash, she doesn't have it in her to torture her innocent nephew with "Reality." Instead, she comes to recognize the sustaining power of Family Connectedness --as she helps construct a "magical protective cave of branches" on the golf course in which she sits with her nephew and sister, holding hands and commiserating as their existence ends.

And the opening sequence which shows us her precognitive Dream of The End actually reveals that she might be wrong about there being Nothingness in aftermath.

The big Clue to that being the case is that, in that opening premonition, Justine is carrying a golf flag --for the 19th Hole. There are, of course, only 18 Holes on a golf course. So perhaps there's another "whole" that she can't understand or "know."