Enola Holmes and the Mark of the Mongoose by Nancy Springer

And she soon finds out that this is not the first of the Earl's wives to die suddenly and vaguely-and that the secret to the fate of the missing Felicity is tied to a mysterious black barouche that arrived at the Earl's home in the middle of the night. The only way forward is for Enola to go undercover-or so Enola decides at the vehement objection of her brother.

The Earl's note is suspiciously vague and the death certificate is even more dubious, signed it seems by a John H. But Letitia Glover is convinced this isn't the truth, that she'd know-she'd feel-if her twin had died. It seems her sister, the former Felicity Glover, married the Earl of Dunhench and per a curt note from the Earl, has died. 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.

At fifteen, she's an independent young woman-after all, her name spelled backwards reads 'alone'-and living on her own in London. But she has all the wits, skills, and sleuthing inclinations of them both.

Enola Holmes and the Mark of the Mongoose by Nancy Springer