The Death of Mrs. Westaway by Ruth Ware

Twenty-one-year-old Hal is all on her own in the world, broke and making a living reading people's fortunes on the pier in Brighton. When she receives a letter from a lawyer saying that she is one of the beneficiaries of her grandmother's will, she is perplexed since her grandparents died years ago. But with a loan shark threatening to harm her, she decides to travel to Cornwall and pass herself off as the Harriet Westaway named in the will. The Westaway home, Trepassen House, is an old manor house in need of repair and the housekeeper, Mrs. Warren is old and creepy, but the family members embrace Hal as their long lost relative. Yet, as she spends time there, questions form in Hal's mind about her possible connection to the Westaways and if she really should be deceiving them. Soon, she finds herself with more questions and suspicions than ever...