The Testament of Sister New Devil

Shinmai Maou no Keiyakusha is a light novel written by Uesu Tetsuto.

Plot
Basara, a teen boy that lives with his parents has to live with two girls he doesn't know, his new step sister. But what he doesn't know is that those step sisters are actually vampires. But he has a secret too. He is a member of the hero clan.

Official site

 * Light novel official site