Shinmai Maou no Testament

Basara Toujou's father suddenly announces that he is remarrying and introduces Mio and Maria as Basara's new sisters. The illusion of a normal family does not last long: Mio is the daughter of the former Demon Lord, Maria is the succubus protecting her, and both are being hunted by forces that want control of Mio's inheritance.

Basara is not an ordinary human either, but a former member of a Hero clan that considers demons its enemies. Instead of handing the girls over, he chooses to protect them and forms a magical bond with Mio that ties their fates more closely together. Between attackers from the demon realm and the Heroes watching him, their new home becomes a defense line for a family that began as an act but grows more real with every danger they face.