The Shifting Shadows of Moongate — Michele Y. Thomas

Ariel Mortow welcomed the chance to spend the summer in elegant Newport when she was commissioned to paint watercolor "interiors" of the Rockwells' fashionable mansion.
But soon after her arrival it became clear that something was terribly wrong. Locked in a loveless marriage from the start, Drew Rockwell and his beautiful, venomous wife were now completely estranged by some terrible secret of the past. A secret that had left their small daughter Blythe screaming in fear at the sight of seemingly harmless moving patterns: the shifting shadows of leaves stirred by the wind; the dancing lights of a chandelier...

Ariel knew she should leave this dark, foreboding house while she still had the chance, but pity drew her to Blythe, and passion bound her to the disturbingly handsome Drew. She was as helpless as they all were against the tides of hatred and tragedy that would sweep them into a vortex of sheer terror.