This is Why We Lied by Karin Slaughter

One toxic family. Eight suspicious guests. Everyone is guilty. But who is a killer?
Welcome to the McAlpine Lodge: a secluded mountain getaway, it’s the height of escapist luxury living.

Except that everyone here is lying. Lying about their past. Lying to their family. Lying to themselves.
Then one night, Mercy McAlpine – until now the good daughter – threatens to expose everybody’s secrets. Just hours later, Mercy is dead.
In an area this remote, it’s easy to get away with murder. But Will Trent and Sara Linton – investigator and medical examiner for the GBI – are here on their honeymoon.
And now, with the killer poised to strike again, the holiday of a lifetime becomes a race against the clock…