Tsuihou Sareta Renkinjutsushi wa Mujikaku ni Densetsu to Naru Yandere Imouto (Oukoku no Shugoryuu) to Issho ni Henkyou de Shiawase ni Kurashimasu!

One day, Mys, the eldest son of Duke Windsor, is sentenced by the king to be exiled to the border. The king claims that his secret visit to Valitra, the strongest dragon that protects the kingdom, is an attempt at treason against the kingdom. However, Valitra's guardian dragon is actually Mys' younger sister, Tinny, who was thought to have died four years ago!