Luce to Shiro no Keiyaku

The fantasy manga centers on Luce, who lives with just her younger sister after their parents died. Their true origin is from an enemy nation, and so the people of their current nation often treat them a bit badly. However, one day someone suddenly appears before Luce, with a strange request: "I want you to become my fiancé‘s lover."

(Source: Anime News Network, edited)