Shinmai Maou no Keiyakusha (新妹魔王の契約者 Shinmai Maō no Tesutamento, officially translated as The Testament of Sister New Devil) is a Japanese light novel written by Uesu Tetsuto and illustrated by Nekosuke Ōkuma. Since 2012, five volumes have been published by Kadokawa Shoten under their Kadokawa Sneaker Bunko imprint. The light novel was adapted into a manga series and has been serialized in Shōnen Ace magazine since May 2013. An anime adaptation has been announced.
Plot
"Hey, you said you wanted a little sister, right?" High school freshman, Basara Tojo, was suddenly asked this question by his father who tells him that he's getting remarried. He then departed overseas after bringing Basara two beautiful step-sisters; Mio and Maria. But the true forms of the two girl's are actually a newbie Demon Lord and a succubus! Basara was almost forced into a master and servant contract with Mio, but a "reversed" contract was formed by mistake, and Basara is now the master. Moreover, Basara is being hit by ecchi situations one after another due to the contract, but Mio's life is being pursued by other demon tribes and hero tribes!! The most powerful contractor's desire action drama begins!