Kokuou ni Natta ga Imouto wa Ore wo Kirau shi, Kokko wa Dai Akaji de Taihen desu

Weisse is suddenly made king after being told that his late mother was a princess. He must cooperate with his sister, the princess, who is not well-liked, in order to rebuild the country from a great deficit. He will make the Princess Rose, full of pride and dignity, into his secret lover, and the two of them will overcome the country's crisis together!

(Source: Bishoujo Bunko, translated)