Auntie Mayhem by Mary Daheim

Though lurching through London is ever so jolly, hostess on-holiday Judith McMonigle Flynn and her cantankerous cousin Renie are looking forward to an unharried weekend at a real English country manor. They find the estate taxing, however, what with vacationing relations crowding every nook and cranny of Ravenscroft House, while its awesomely aged mistress, Aunt Petulia, holds court—until a box of poisoned sweets hastens the dour dowager's demise. Soon Judith and Renie are up to their American necks in a muck of murder most British—as they set out to unearth a fatal family secret...and unmask the culprit who was anti-Auntie enough to do the old girl in.