The Sanatorium by Sarah Pearse

An imposing, isolated hotel, high up in the Swiss Alps, is the last place Elin Warner wants to be. But she’s taken time off from her job as a detective, so when she receives an invitation out of the blue to celebrate her estranged brother’s recent engagement, she has no choice but to accept. Arriving in the midst of a threatening storm, Elin immediately feels on edge. Though it’s beautiful, something about the hotel, recently converted from an abandoned sanatorium, makes her nervous – as does her brother, Isaac. And when they wake the following morning to discover his fianc?e Laure has vanished without a trace, Elin’s unease grows. With the storm cutting off access to and from the hotel, the longer Laure stays missing, the more the remaining guests start to panic. But no-one has realized yet that another woman has gone missing. And she’s the only one who could have warned them just how much danger they’re all in.