Imouto Dakara Dekiru Koto, Imouto Janai to Dame na Koto.

"Playing siblings with a complete stranger... Then there's no need for self-restraint, right?" The girl I just met said it cheerfully with a carefree smile, as if she were just an old friend inviting me for a detour on the way home from school. With lovely mannerisms, like reciting from a favorite poem collection, the girl lured me. Me, who was in love with my real sister. Me, who was lusting after my real sister. The girl, who knew my secret that should never be revealed to anyone, hinted at a honey-like threat. "I'll do what you can't do with your real sister." And so, that day, I pinned her down. A one-time mistake. A one-time compromise. A moment of intimacy. That was how our relationship was supposed to end. That was how we were supposed to go back to being strangers. But, at the time, I had no way of knowing. "I'm happy to meet you again, (step) brother." The girl I thought I would never see again ended up becoming my step sister. This is the story of the worthless me who's in love with my real sister, and my troublesome step sister who's too attached to me. And of an ordinary little sister who's bothered by our relationship. I'm sure that's all there is to it. Probably. [Translated from Digiket]