Shinmai Maou no Testament  

Shinmai Maô no Testament, Shinmai Maou no Keiyakusha, The Testament of Sister New Devil, 새 여동생 마왕의 계약자, 新妹魔王の契約者[テスタメント], 新妹魔王的契约者, 新妹魔王の契約者〈テスタメント〉

When Basara Toujou meets his new stepsisters for the first time, it's not exactly a great start. His father introduces two gorgeous girls as his newest siblings - Mio, a busty redhead with long silver hair, and Maria, a petite blonde with short black hair. But things take an unexpected turn when Basara discovers that these seemingly normal sisters are actually demons: Mio is the daughter of the former Demon Lord, and Maria is her trusted succubus servant. When Basara learns about their true identities, he realizes that his own past as a member of a clan of "Heroes," sworn enemies of the demons, comes back to haunt him. Despite this revelation, Basara decides to protect his new sisters and forms a master-servant contract with Mio to keep watch over her. As he tries to keep his family safe from hostile demons, Basara must also confront his own dark secret that still weighs heavily on his conscience. With the Heroes watching his every move and the constant threat of danger looming over him, Basara must do everything in his power to protect those he cares about.

Shinmai Maou no Testament adapts the first 3 volumes of Tetsuto Uesus light novel series of the same title.