Onee-chan ga Ecchi na Koto bakka Suru kara...

Despite his desperate protests that he already has a girlfriend, a young man finds himself entirely at the mercy of his older sister's relentless and inappropriate advances. What begins as an awkward domestic situation quickly spirals into a deeply compromising dynamic, as she refuses to respect his boundaries or acknowledge his existing relationship. Her fixation on him transforms their household into a place where he can no longer find any semblance of normalcy or privacy. Stripped of his ability to resist her overwhelming persistence, the protagonist is gradually reduced to nothing more than a convenient outlet for his sister's insatiable desires. His attempts to maintain his romantic life outside the home are constantly undermined by her manipulative behavior behind closed doors. The narrative explores this taboo power imbalance, chronicling how an ordinary boy is systematically broken down and repurposed solely for her physical gratification. Caught between the girl he genuinely cares for and the sibling who dominates his every waking moment, he is forced into a secret double life he never asked for. Ultimately, this provocative story follows his complete loss of autonomy, detailing the inescapable reality of becoming his older sister's personal plaything against his will.