April's Grave / Call in the Night (Vintage Hardcover) by Susan Howatch

An intriguing story of Karen Bennett, whose twin sister, April, has been missing for three years- ever since Karen's shocking discovery of her sister's love affair with her husband, Neville Bennett Now, after a long estrangement, Karen and Neville have reconciled, both determined to forget the beautiful selfish woman who nearly destroyed their lives. But April's shadow continues to haunt them.... and Karen begins to suspect that her sister is not just missing but that she is dead.... murdered!