Each protagonist has a sibling that is (or was) close to them: Connor has a twin sister, while Gwen has a brother who is barely a year older, a brother who has become more distant and hostile since they moved into the house. Meanwhile, ten years earlier, Connor had a similar temperament, giving into dark thoughts and violent urges, seeming to feed on the evil energy of the house while simultaneously it fed on him.
The parallels between the two stories grow more evident as the story continues, and then things begin to line up, overlap, and explode.
The dual narrative definitely works in this scenario, with Gwen's fear that she is going crazy (again) contrasting effectively with Connor's unapologetic enjoyment as he embraces his darkness. Gwen thinks she's pathetic, but readers will find her sympathetic; Connor is twisted, and he likes it that way. It is interesting to note that both characters are trying to be happy in their own ways and both are pretending to be something they're not.
With short, unnumbered chapters - sometimes no longer than three-quarters of a page - the action moves quickly, and the format and plotting of the story should attract and intrigue horror fans, even those with shorter attention spans.