The League of Dangerous Young Ladies by J.A Morgenstein

It's 1909 and Rose Moriarty—teenage daughter of Sherlock Holmes' greatest enemy—has made a name for herself fighting monsters and solving crimes. But that was before Rose met the one mystery she couldn't solve: the disappearance of her headmistress. Now, her school has shut down, her classmates have scattered, and Rose is on her own.

On the very day Rose receives word that an old friend is dying, the shadowy Count Christoph and his ward Clara show up at her door. Can Rose convince these strangers to help save her friend's life?