Still See You Everywhere by Lisa Gardner

From #1 bestselling author Lisa Gardner comes a harrowing new thriller: Frankie Elkin is an expert at finding the missing persons that the rest of the world has forgotten, but even she couldn’t have anticipated this latest request—to locate the long-lost sister of a female serial killer facing execution in three weeks’ time.

Frankie Elkin is an expert at finding the missing persons that the rest of the world has forgotten, but even she couldn’t have anticipated this latest request—to locate the long-lost sister of a female serial killer facing execution in three weeks’ time.
She has called herself “death,” but people called her the devil.
The case was sensational. Kaylee Pierson had confessed from the very beginning, waived all appeals. Despite the media’s chronicling of her tragic circumstances—the childhood spent with a violent father—no one could find sympathy for “the Beautiful Butcher” who had led eighteen men home from bars before viciously slitting their throats.
Now, with only twenty-one days left to live, Pierson has finally received a lead on the whereabouts of the sister who was kidnapped over a decade ago, and she needs Frankie’s help to find her. The Beautiful Butcher’s offer:
When was the last time your search ended with finding the living?
Unable to resist the chance for a rescue, Frankie takes on Pierson’s request. Twelve years ago, five-year-old Leilani went missing in Hawaii. The main suspect? Pierson’s tech mogul ex-boyfriend, Sanders MacManus. Now, on a remote island in the middle of the Pacific—the site of MacManus’s latest vanity project—fresh evidence has appeared. In order to learn the truth and possibly save a young woman’s life, Frankie must go undercover at the isolated base camp. Her challenge: A dozen strangers. Countless dangerous secrets. Zero means of calling for help. And then the storm rolls in…