Ane no Kawari ni, Yomei Ichi-nen to Iwareru Koushaku Shisoku-sama to Konyaku suru Koto ni Narimashita

Edith is an orphaned girl who is treated coldly by her stepparents and stepsister. One day, her stepsister receives a marriage proposal from Lionel, the eldest son of the highly prestigious Count of Granvelle. However, there is actually a hidden story behind this proposal... and Edith is suddenly engaged in place of her stepsister. "I'm sorry, you can't be engaged to someone like me. Can you just bear with me for a year?" Her fiancé only has a year left to live, and this is a happy love story in which pure love creates a miracle!

(Source: Kodansha, translated)