My Older Sister is the Main Character

When Helena was reborn into a novel, she was forced to grow up knowing that her sister was the main character and that a terrible life awaited her… unless Helena could stop it from happening. By learning how to take care of family matters, Helena took her sister, Callisto’s, place as the head of the house, allowing her to live her dream of being a knight. But she now finds herself hurtling towards both the terrible fate once meant for Callisto and the man who makes it come to pass.

(Source: Tapas)