Spindrift — Phyllis A. Whitney

Inside the wall of a palatial Newport estate, a violent struggle was beginning. Christy Moreland had come back to Spindrift determined to uncover the truth behind the mansion's most tightly held secret: the reason for her father's death.
It was a truth that, at times, Christy felt she was barely strong enough to face, for it meant openly confronting Spindrift's domineering mistress-her mother-in-law, Theodora Moreland.
Yet it was a battle that Christy knew she could not escape, especially if she was to free her young son from the obsessive and destructive hold the Morelands had come to exert over him.
As Christy searches out the mysteries that haunt Spindrift-and the people in it-she slowly unveils a tightly woven web of deceit that only murder can continue to conceal....