Imouto Twins (2006)

Twins Izuru and Kokone harbor a deep love for their elder brother, but only Izuru has acted upon her feelings so far. When she arrives home from school, Izuru often goes directly to her brother’s side and indulges in him. Unlike her proactive sister, Kokone merely watches them from the side while drowning in jealousy. All she can do is pleasure herself to the sound of their salacious acts. One day, however, Kokone’s frustration bubbles up, leading her to lash out at her sister and brother. Now, Izuru takes it upon herself to console Kokone, trying to convince her to be more honest about her feelings.