The Last Daughter of Highdown Hall by Cathy Hayward

She loved her mother, but did she really know her?

As their mother Milly lies dying, the four Highdown Hall sisters gather at her bedside. For months, the youngest daughter, Zoe, has cared for Milly in the family home. The older sisters all have homes of their own, so Zoe will inherit the crumbling manor house and continue their mother’s legacy.
But then they read their mother’s will: Zoe is cut out of Highdown Hall entirely—the house goes to her sisters. Zoe is utterly shocked and devastated—did she not know her mother at all?
With tensions rising between the sisters, Zoe begins sorting through Milly’s belongings, trying to find something to help her understand. When she discovers a mysterious iron key that fits no door, she is left only with more questions. What was this terrible secret her mother could never speak about?

As Zoe begins to dig into the past, she tries to hold onto her memories of the mother she loved. But what could have been so devastating that Milly kept silent her whole life? And if Zoe uncovers the truth—will it destroy her forever?