I'm not sure where to put this. It's not a horror story and if you read the afterward it's not considered one. It's a creepy story right up until you get to the part where you realise this is kind of just "The Good Son", which I hope isn't that much of a spoiler but come on, how many people are going to remember that movie unless they saw it - and there wasn't that many people who saw it, it only made $60M - so I'm good with the reference if you are.
The story is very carefully plotted and the way information is given to you is through an unreliable narrator. But also she isn't? I mean the second sentence of the story is "I am eighteen years old, and I live with my sister Constance." That's clear from the jump and yet I spent the entire rest of the story imagining her much younger because of how she is shown and she's the one doing the showing. So how can you take her seriously when she is clearly addled in some way?
I don't remember the moment that I thought to myself, "Ah ha, the underlying story/mystery that keeps coming up constantly in fits and spurts is actually _____" but I do know that suddenly a lot more things made sense. The fact that Constance had a weird agoraphobia though I never understood. The afterword says that it's one half of Shirley Jackson's life because she was diagnosed with it late in life but that's too easy, that's just weird story motivation and reading something in real life and making a leap to conclusions mat.
You know, Office Space? ah well.
The book has so much hiding around the corners that you are never given, only hinted at. I suppose this is what makes a compelling short story/novella, the ability to give you a world that feels lived-in in so few pages. It's not just the story that's happening to you at the moment but also the story that you can barely see at the edges. Kind of like in "The Ring" when they find the extra film on the film edge.