Kijoku: Princess Double Gari

Distol El Asteria, prince of Asteria, endured 8 humiliating years as a hostage in Baltrage following his mother's miserable death during captivity. He closes his heart and trains intensely in swordsmanship and knowledge to protect loved ones. Despite his royal status, he's treated as a nuisance except by Baltrage's first princess, Irina. When he learns his adopted sister Kuuna is chosen as a sacrificial "saint" to seal the demon king, Distol despairs over his powerlessness to protect her. A mysterious girl, claiming to be a witch, suddenly appears before him and offers immense magical power to save Kuuna in exchange for invading Baltrage and corrupting Irina. Distol, torn by the conditions but desperate to save his sister, accepts, knowing it will bring chaos and potentially ruin him. [Condensed from DLsite]