Koishita Hito wa, Imouto no Kawari ni Shindekure to Itta. Imouto to Kekkon shita Kataomoi Aite ga Naze Imasara Watashi no Moto ni? To Omottara @comic

The noble lady Wisteria was in love with Bright, the beautiful and kind son of a duke. She was determined to become engaged to him and made every effort to do so, despite her bad reputation. However, it was her adoptive younger sister who Bright opened his heart to. One day, her sister was chosen to be a sacrifice. As Wisteria devoted herself to helping her avoid her fate, she discovered Bright's feelings. Eventually, Wisteria was told the decisive words, and she was sacrificed in place of her sister. She should have lost her life there, but...

This is the story of a wounded and lonely former noble lady and a young man who was blessed with everything and had no attachments.

(Source: Shousetsuka ni Narou, translated)