The testament of sister new devil

High school student Basara Toujou's father remarries and he gains two little stepsisters. However, when his father leaves on a business trip, Basara learns his sister Mio is heir to the Demon Lord, and the other sister Maria is Mio's succubus attendant. Basara offers to help the sisters escape demons, but when he enters into a magical bond with Mio to be her servant, something goes wrong and he becomes her master instead. Now he must force her body to submit to him to break her curse.