愛妹恋愛ケッコンしたい！ オトメの二乗

Based on the erotic game by Interheart. Shun'ichi, a university student, has two younger sisters. After their parents passed away, he lived as their elder brother and guardian, but one day he witnessed the eldest sister, Emi, masturbating, and their relationship began to change dramatically. Meanwhile, the second sister, Minami, mistakenly believes this is a trick to marry her beloved brother, and masturbates, awakening to sex. Shun'ichi, who is now aware of his two younger sisters as the opposite sex, crosses the line between brother and sister... (Source: parabook, translated)