When Katharine "Katy" Devlin turns up dead, there are tons of murder suspects. Who would want to kill a twelve-year-old girl who wants to be a dancer? Apparently, we need only look as far as her psychopathic sister, Rosalind. She's such a psychopath, though, that she has a page all to herself. Here it's all about the rest of the Devlin clan.
Of course, Katy's family is the primary suspect at the beginning of the investigation (although Rosalind is close to the bottom of the list). Margaret, the mother, is a sobbing mess, and there are suspicions she may have been making her daughter purposefully sick. Of course, it turns out she wasn't and that Rosalind was actually the one poisoning Katy—so sorry, Margaret.
Jonathan, the father, is suspected of murder, as the father always is. He didn't do it either, but his connection to a gang rape in 1984 doesn't help his case. However, in the end, the accusations of child abuse stick with him, and he has to move his family away to escape them. Plus, he lives in fear that Rosalind will continue her manipulative ways—he just knows she'll never change. So much so, that he even wishes his wife had had an abortion when she was pregnant with the girl (25.54). Yikes.
There's a third Devlin daughter, Jessica, Katy's twin, who has some sort of developmental disability and is often manipulated by Rosalind. Rosalind gets Jessica to corroborate the whole man-in-a-track-suit story, for instance, keeping the detectives' eyes on a different prize.
For the sake of completion, we have to mention Auntie Vera Foley and her children, Katy's cousins, who live in a house that "smelled of cabbage" (7.21). Her daughter, Valerie, is the only member of this family who doesn't lie, yet Vera constantly tells her to stop talking.