Soon She Will Be Gone — John Farris

To the outward eye the billionaire Trevellian family appears to be the paradigm of culture and class. But when hot young artist Sharan Norbeth is taken under their wing, she discovers that family loyalties have become horribly twisted, and hidden evil festers below the surface. Sharan must pry apart the mystery hidden in the family's innermost core - and fast, if she doesn't want to disappear into a traceless oblivion.