Then Let Be the Bad Lady.

Countess Aldante, treated like a servant by her family, is forced into the harem of the notorious womanizing First Prince through her stepsister's machinations. However, Aldante is clever, strong, and more of a "villainess" than anyone else. In fact, Aldante was a notorious villainess in high society until three years ago. For reasons unknown, she had been hiding her true identity, but rather than ruin her life, she'll return to her villainous ways! "I will get revenge on those fools who harm me, twofold!" -- Her oppressive family, her fiancé who is having an affair with her stepsister... and the entire corrupt country will be mercilessly crushed by this ultimate villainess!