Migawari Hakushaku

Mireille is the daughter of a bakery owner and her twin brother Fredrick was adopted into another family when they were kids but they remained in contact with each other by sending letters to each other. One day a letter from her brother said he was in love with another man's woman. She replied, provoking him to be a man and claim the woman as his. Two months later, a beautiful young man named Richard, claiming to be a messenger from her twin brother Fred, visits and tells her "I've come for you." When the bewildered Mireille resists, he proceeds to kidnap her! She learns that Fred has run and eloped with the crown prince's fiance and now, it is now her responsibility to impersonate Fred who was the former Count of Bernhardt! Source: My Anime List