The Heiresses Books
The book in The Heiresses series by Sara Shepard (2014).
About The Heiresses
The Heiresses is a standalone adult thriller about the Saybrook family, Manhattan socialites who seem to have it all. But a string of mysterious deaths, nicknamed "The Saybrook Curse," haunts the family. When another Saybrook heiress is murdered at a family party, the surviving women must figure out who among them is the killer.
This was Sara Shepard's first novel written for an adult audience, blending her signature twists with a more mature setting.