Enola Holmes Series By Nancy Springer :and the black barouche

Enola Holmes is the much younger sister of her more famous brothers, Sherlock and Mycroft. But she has all the wits, skills, and sleuthing inclinations of them both. At fifteen, she's an independent young woman--after all, her name spelled backwards reads 'alone'--and living on her own in London. When a young professional woman, Miss Letitia Glover, shows up on Sherlock's doorstep, desperate to learn more about the fate of her twin sister, it is Enola who steps up. It seems her sister, the former Felicity Glover, married the Earl of Dunhench and per a curt note from the Earl, has died. But Letitia Glover is convinced this isn't the truth, that she'd know--she'd feel--if her twin had died.