Mistress of Darkness by Hazel Hunter

Gwen Archer does not fear the curse of Dredthorne Hall. Instead it is the message she must deliver that gives her pause: she will inform her future brother-in-law, Christopher, that her sister has run away.
But what Gwen discovers at the Hall is far worse than being the bearer of bad tidings. She finds Christopher’s older brother, the reticent and rude Robert Sheraton, there to greet her. Now she must impart her unwelcome news to him instead.
To her shock though, the man that she meets is not the Robert that she remembers. Instead he seems the perfect host. But as her stay at the hall lengthens, and she and Robert draw close, strange accidents beset her. Soon she must watch every shadow and listen at every door, lest the Ghosts of Dredthorne envelop her in their dark embrace.