Suki na Ko ni Kokuttara, Futago no Imouto ga Omake de Tsuitekitta

One day, when they happened to be alone in the classroom after school, Nakaba Masaki suddenly confessed love to his classmate, Yuzuki Tsubasa, He was prepared to be crushed by her words of rejection, but what he received was a surprising reply: "I'll go out with you, if you'll play along with two-timing with my twin sister."
For a moment, he didn't understand what she was talking about, but he knew he couldn't pass up this opportunity, so he agreed to go out with both of them at the same time. But even more unexpected events awaited him!
(Source: Brave Bunko, translated)