The Cornish Inheritance by Fil Reid

After her father’s suicide, Caroline Fairfield is left practically destitute and needing to find herself employment in a hurry. Her old friend, Ysella Beauchamp, now living in Cornwall, assures her that a family she knows is looking for a governess for nine-year-old Yves, the orphaned heir to his grandfather’s title and fortune, after the previous governess left under a cloud.
He’s described as weak and delicate, but turns out to be anything but, and Caroline soon discovers that Mrs Treloar, Yves’s formidable aunt, has a distinct lack of concern for his welfare. To the point of seeming to want to put him in danger. And then she discovers he’s being dosed every night, on his aunt’s orders, with something that could prove fatal.
Major Nathaniel Treloar, disfigured in battle and returning from Europe now Bonaparte is in captivity, travels down to Cornwall to recuperate at his grandfather’s estate while he decides what to do with his life. He’s touchy about his damaged looks and, although he seems friendly enough, and she’s strongly attracted to him, Caroline can’t get out of her head the fact that he would be the one to benefit if anything were to happen to Yves.