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18+ Suki na Ko ni Kokuttara, Futago no Imouto ga Omake de Tsuitekita

On a summer afternoon, Nakaba Masaki suddenly confesses his feelings to his classmate Yuzuki Tsubasa, who happens to be alone with him in the classroom after school. He braces himself for the inevitable rejection... but the answer he receives is nothing he could have imagined: "I'll go out with you, if you're willing to date my twin sister too."

(Source: Hifumi Shobo, translated)