One Day, My Sister Died

My sister, who was practically sold off from the Pheliese mansion to become the bride of the monstrous, dethroned crown prince, has returned as a corpse. And now... I am next. I am to be the ninth princess of the dethroned crown prince, and I am heading to the black mansion. "After our wedding night?" "If we spend our wedding night together, does that mean you cannot cast me out, even if you wish to?" I will survive in this dark mansion, where the monstrous prince who may have killed my sister resides, and I will uncover the truth of her death. No matter what it takes.