Mujikaku Seijo wa Kyou mo Muishiki ni Chikara wo Tare Nagasu: Koushakuke no Ochikobore Reijou, Totsugisaki de Shiawase wo Tsukami Toru

Lady Carolina Sanchez has spent her entire life believing she is the "Sanchez family disgrace," a notion constantly reinforced by her elder sister, Flora. In public, Flora is a model of kindness and gentility, always coming to her baby sister's defense...but behind closed doors, she heaps abuse on Carolina, leveraging Carolina's perceived shortcomings to suit her own ends. When Carolina is sent to the neighboring Malcosian Empire for a political marriage to Prince Edward Ruby Martinez, she expects to disappoint her new family too. But, to her surprise, her husband-to-be and her new in-laws seem to genuinely adore her! Can Carolina escape the anguish of her past and rise to the occasion as Edward's imperial consort? She just might, especially if she's not quite as powerless as she assumed...

(Source: J-Novel Club)