Aneki no Kounai Kaikinbi

Upon discovering her younger brother uses her undergarments for his personal gratification, 23-year-old Anna Hirose resolves to proactively manage the situation. She intentionally begins revealing her own provocative behavior to him, progressively intensifying her displays. She later formally declares that, as his elder sister, she will address his sexual inquisitiveness directly. She establishes a single prohibition against vaginal intercourse but permits all other acts. As his demands grow increasingly explicit, she upholds her pledge, accepting full responsibility for her decision to continue.