S-kei Kikoushi no Okiniiri

My sister is the bride of a count and I'm to be his mistress? "Your life in this family is so sad. You have to be bedded by a man you don't even love," Elliott whispered in Laurel's ear as he continued to take her night after night. He was a count who was engaged to Laurel's younger sister. However, her sister was in a relationship with another man. When Laurel covered for her, she ended up agreeing to become Elliott's mistress to keep his attention on her. Laurel must have her body and heart played with by him to repent for her sister's sin. She shouldn't enjoy it, but his intimate touches make Laurel feel almost as if he loves her... (Source: MangaPlaza)