I Abandoned My Engagement Because My Sister is a Tragic Heroine, but Somehow I Became Entangled with a Righteous Prince (Manga) Vol. 3

Living in the royal palace as Prince Eric's guard, Leia finally has a place to come home to where she feels safe and heard. However, she discovers that her sister Jill is cheating on her fiance with Duke Berklein, the very duke who wants Eric dead. Not only that, but further investigation reveals that Jill is dabbling in Death Poison! Will this revelation finally end Jill's perfect "tragic heroine" sister act?