The Single Musketeer - The Thrilling Adventures of the Most Dangerous Woman in Europe, #10

To a child, the world is a bright and innocent place. But while their famous mother Marian Halcombe is in Asia, William Camlet and his older sister Lester take turns recounting an adventure that took place 25 years ago, when they were adventurous children. And now they discover it was more dangerous than they knew. Treason, a duel to the death, blackmail, a bride compelled to the altar, and the recovery of an Egyptian treasure lurk in the past.
But the lessons and weapons of childhood are the foundation for adult valor. William finds love, and Lester sets out on a quest for the biggest villain of the 1890s: Jack the Ripper!