The Nightingale Trilogy by Cynthia Dane

Nala’s sister is dead. The man who had her killed hides within a sinful, hedonistic refuge that she can’t penetrate on her own.
Up and coming billionaire Vincent Lane offers her an in: become his sub and join pharmaceutical giant Xavier Crow’s dangerous world of sin, scheming, and a secret circle known only as “The Aviary.”
Nala must don the codename Nightingale. Small. Sweet. Beyond determined. Her sister’s memory demands nothing less.
But she can’t get distracted by Vincent’s powerful façade. Not until she discovers what dark and terrible secret he’s hiding as well.