Many people didn't like this short story because of it's cliffhanger ending, however I think it's perfect.
The setting of two siblings living together is quite unique, and how they avoid the rooms that were taken over was, in a way, funny and relatable.
I believe some people feel frustrated by the ending, but I think it's good to have a mysterious end, because it makes us think, and be invested in the story. Unsolved murders are popular because it's unsolved. When it is solved we get disappointed in some cases, and I believe it would have been the same if the author revealed what the "thing" was. The mystery is the story.
(Also, I'm just going by perspiculation, that the people didn't like this story because they did it at school... Which does ruin the enjoyment.)