Dahaenginji Bulhaenginji

The spunky Lady Dillion didn't realize she was the villain of a romance novel until she encounters Marquess Cedric. Seeing him jogs a dormant memory of the novel's plot, in which her beloved younger sister dies in childbirth after marrying Cedric. Determined to prevent this, Dillion marries him herself, but what fate awaits her when she knows he's meant to fall in love with the real female lead later? In this marriage of convenience, it seems nothing turns out quite how either of them expects...

(Source: Tapas)

Note: Includes 18 extra chapters.