Most Secret - Kathleen Buckley

Jane Stowe frequently finds her irritable father, peevish stepmother, and half brother Rupert a trial. Her only hope of eventual escape is her maternal uncle, Roger Markham, whose heir she is. When he dies under mysterious circumstances, Jane is the obvious suspect.

Alex Gordon, family misfit, has been sent to find out if there's anything to Markham's suspicions about the schooner Sea Mew. With half the Continent at war, and the Young Pretender recently landed in Scotland, the matter may be of critical importance. Once Alex ferrets out—with Jane's assistance—the connection between the Sea Mew and Jacobite activity, he is told to leave the rest to the professionals. But the professionals have no stake in saving Jane from the gallows or Rupert from a charge of treason.