Becoming the Villainous Duke's Family

Story

Lowena, a sweet but terminally ill orphan girl, holds a secret: her world exists within a novel, and only one person is capable of finding a cure for her otherwise untreatable disease. Unfortunately, that person happens to be the Duke of Somerset, the ruthless and obsessive male lead of the story. With her days numbered, Lowena decides to infiltrate his household with a genius plan... become family to the widowed duke and his nephew and win their hearts before the illness stops hers!

Synopsis via AniList (English).