The Testament of Sister New Devil

"Hey, you said you wanted a little sister, right?" First-year high school 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. But Mio and Maria's true forms are actually the newbie Demon Lord and a succubus!?