Boksuui Sijageun Gyeolhonbuteo

Story

On the day I was betrothed to the Crown Prince, I witnessed my father and the Empress in a compromising position. For my mother’s sake, I tried to pretend I hadn’t seen anything and endure it. But after my mother discovered the truth and took her own life, all that was left for me was revenge. To deal with these madmen, don’t I have to become a madwoman myself? “Your Royal Highness, the Crown Prince, will you marry me?” The day before the wedding, to take revenge on the Empress and my father, I proposed to my fiancé’s half-brother.

Synopsis via AniList (English).