Shinmai Maou no Testament

Japanese Title: 新妹魔王の契約者（テスタメント）

Plot
"Hey, you said you wanted a little sister, right?" First year highschool student, Toujo Basara, was suddenly asked by his father and fell into a state of panic. On top of that, the eccentric father told him he will be getting remarried.

He then departed overseas after bringing Basara two beautiful step-sisters. Basara met his two cute little sisters, Mio Naruse and Maria Naruse. But Mio and Maria's true forms are actually the newbie Demon Lord and a succubus!?

The two sisters revealed themselves as a member of the demon clan and Basara as a member of hero clan, demon clan's enemy, and is not related by family. Mio is the future Demon Lord and since Mio's power is not fully awakened, Basara will protect her from the demons who are after the power she inherited from her deceased father, Demon Lord Wilbert.