The Case of the Missing Marquess by Nancy Springer (2006) Philomel Books

Enola Holmes is introduced as the young sister of Sherlock and Mayfield Holmes.  Her mother has disappeared, and the brothers expected a full staff at their estate.  But there is only a Butler, his wife, the cook, and their son.  The brothers intend to send Enola to a boarding school, which appalls her so, she runs away – to London, and solves the case of the kidnapping of Tweksbury, who was not kidnapped, but ran away himself.  Only Enola gets kidnapped by the same two men who kidnapped Tweky after he arrived at the docks of London hoping to get on a ship for adventure.  Will she get away, will she save Tweky, will she evade her brothers?  And why didn’t her mother take Enola with her?