At the end of the game, it'd be nice if you added something to indicate that it ended. I assume the sister disappeared, so for example, the brother could wake up and find his sister gone and says, "sister, where are you?". Or maybe the sister can scream before she disappears.
But overall, the story is quite interesting, I think some more sound effects can be added to give it more life, even though there's no more life haha. So something like raining sounds and maybe some wind to show how empty it is.
Good job on the game!
Here's the clip where I played your game: https://www.twitch.tv/videos/2520907924