The Night Dancers by Jude Knight

Disguised as an investigator, bound by secrets, united against a monster—they’ll either expose the truth or be destroyed by it.

Certain that the Marquess of Teign is behind her cousin’s disappearance, investigator Melody Blackmore enters his mansion disguised as a man. Teign hires her to discover how his sons are leaving their tower prison or having food and other items brought in, and she soon realizes that the sons are also the marquess’s victims. As her interest in the eldest of the brothers grows, she joins them all in a campaign to bring Teign down.
Allan Sheppard, the Earl of Kemble, is the eldest of Teign’s ten sons. He is weighed down by his frequent failures to protect his brothers from Teign’s beatings and abuse but determined to keep them as safe as he can until his youngest brother is no longer under Teign’s guardianship.
All they must do is fool the most recent investigator sent to find out their secrets. But Mel Black is not like the others, and Allan finds that an alliance with her gives the brothers the chance to not only survive, but to thrive.

However, Teign will stop at nothing to punish his sons for escaping him. Only Allan’s and Melody’s growing commitment to one another keeps them steadfast as they uncover evidence of unspeakable crimes and evil beyond imagining.