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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!

"Mys Nii-sama is the one who saved my life, the one who is worthy to rule everything in the world. Anyone who hurts Mys Nii-sama will be thrown into hell!"

And the kingdom is heading towards destruction, having incurred the wrath of its guardian dragon, Valitra... Meanwhile, on the border where Mys has become the ruler, his alchemy techniques are leading to unprecedented developments and making everyone happy—and once again, the kingdom is in danger of destruction.

(Source: Takeshobo, translated)