The Case of the Missing Marquess (Enola Holmes, #1) by Nancy Springer (2006)
When Enola Holmes, sister to the detective Sherlock Holmes, discovers her mother has disappeared, she quickly embarks on a journey to London in search of her. But nothing can prepare her for what awaits. Because when she arrives, she finds herself involved in the kidnapping of a young marquess, fleeing murderous villains, and trying to elude her shrewd older brother—all while attempting to piece together clues to her mother's strange disappearance.