Ore no Imouto ga Saikou no Okazu Datta: Cool na Kanojo ga H no Toki Dake Dereru Riyuu

"It seems my brother is a complete pervert." Those are the words that shatter the protagonist's privacy when his own sister unexpectedly catches him in the act of masturbating to an adult incest-themed video game. Rather than recoiling in disgust or reporting his deviant behavior, she makes a shocking decision: she chooses to help him out herself. To his absolute disbelief, she initiates physical contact using her mouth, crossing a boundary that transforms their ordinary sibling relationship into something entirely taboo and irreversible. Despite constantly scolding him for his insatiable and lustful desires, the sister willingly engages in increasingly intense and explicit encounters with him. What begins as hesitant teasing rapidly escalates through a series of provocative scenarios, ranging from playful cosplay outfits to passionate oral acts and unprotected climaxes. Her outward disapproval sharply contrasts with her active participation, creating a confusing yet intoxicating dynamic between the two siblings. As their secret indulgences grow more frequent and extreme, the true nature of her involvement remains deeply ambiguous. She claims her goal is to cure him of his sexual perversions once and for all, but her escalating willingness to participate suggests otherwise. Caught between her sharp tongue and her eager actions, the protagonist is left to wonder whether his sister is genuinely trying to rehabilitate him, or if she has become the very source of his deepest desires.