Murder at the Castle by M.B.Shaw
Murder at the Castle is the second in the Iris Grey series, and while it can certainly be read as a stand-alone, events from the previous book are referenced. The author gives enough information that a new reader wouldn't be lost, but since that case is why Iris is in Scotland and does affect how she views things, I think it's worth reading.
Iris is commissioned to paint a portrait of Kathy Miller, a beautiful American due to marry a wealthy elderly Baron, Jock MacKinnon. MacKinnon's adult children dislike Kathy intensely - she displaced their own mother in the castle and she's much younger than the Baron. The townsfolk don't like her because she has some new ideas about how things around the castle should be run - and she's American. Kathy's been receiving threatening letters, which she wants Iris's help putting a stop to. There's already plenty of tension before the two women's bodies are found buried on the castle...