The Testament of Sister New Devil

Basara Tojo is a high school student studying at Hijirigasaka Academy when his father Jin suddenly gets "remarried" and asks Basara if he wants to have sisters. Basara meets his two new, cute stepsisters, Mio Naruse and Maria Naruse. After his father departs overseas, Basara quickly finds that the two sisters are actually part of a demon clan, and he is meant to be with the hero clan. Mio, a Demon princess, forcibly attempts to make a master and servant contract with Basara, but Maria has it end up being the opposite, with Basara as the master. Basara then keeps getting into ecchi scenarios, largely to protect Mio from being pursued by other demons who are after her heritage, which is passed down from the former Demon Lord Wilbert.