Ward, Sarah (British author). Mallory Dawson #1

Mona is a carefree artist staying at the island's upscale hotel to work on her illustrations. Beth is the harried mother of a toddler, on the island for a weekend escape with her family. Charlotte wanted a romantic getaway with her husband, not a trip with his troubled teenage stepdaughter. One of them is a serial killer who poisoned four of the girl's friends at her eleventh birthday party. Three guests who fit the profile. Cut off from the mainland and isolated on a remote Welsh island, former detective-turned-hotel-manager Mallory Dawson will have to figure out whodunit before the killer risks everything to attack again