Game Kaishi Mae ni Shinu Mob Akuyaku Ouji ni Tensei shita Ore: Oshi Heroine no Imouto to Shiawase ni Naru Tame ni Saijaku Mahou "Dark Edge" wo Kajou na Doryoku de Kiwamenuitara, Saikyou no Bukkoware Seinou to Kashiteita Ken

Luke is reincarnated as a minor villain prince from a game he played in his previous life, a character destined to die before the main story even begins. In this world, he has an adorable younger sister named Diana, whom he cherishes above all else. However, Luke knows a terrible secret: Diana is fated to fall into darkness and ultimately die. His beloved sister ending up corrupted and dead? There’s no way he’ll allow that scenario to happen! Determined to defy fate, Luke begins training relentlessly from the time he is a baby. By pushing the supposedly weakest magic, “Dark Blade,” to its absolute limits through overwhelming effort, he gradually turns it into a monstrously overpowered ability. This is the story of a mob prince striving to become the strongest in order to protect the sister he loves most.

(Source: Kodansha, translated)