Since this came up a few times, let me just tell everyone that this oneshot has no twist. Not one. And I'm not saying this because it was badly done or too obvious. There was no plot twist planned from the beginning. No halfway intelligent Japanese reader would have been surprised by the "reveal" that the sister was dead. That wasn't the point of this. The first big clue was the firefly, which has a significance in Japanese folktale. Perhaps you've heard of the anime, Grave of the Fireflies. The firefly leading Yousuke to the sister was the first big clue. In fact, too obvious to be called merely a clue. And right after that, the sister (and the readers) see the floating lanterns and realizes what day it is. So it's established right from the beginning that she's dead. And if you predicted that "twist," pat yourself in the back for being sharp enough to see that water is indeed wet.
This is instead a narrative about what changes and what does not, and how that affects and allows us to hang on and then let go. In that, the author does as very good job of presenting foreshadowing and blending flashbacks while keeping the story tight enough for us to wonder whether the brother is actually seeing a ghost or he's simply hallucinating or imagining. And then the author wraps it up with the final epiphany that allows the boy to confront reality being the same as the one the sister had earlier. That's good storytelling.
I'll give you that the theme is old and common. But that's just another way of saying that it's timeless, and you can still weave a great story with it. It's much better than bungling a fresh, new idea into something trite and empty.
edit:
@BraveDude8.
Don't fuck with my post without even talking to me.