Izou Kibayashi

The joys of summer break; warm weather, sun soaked beaches, time off from school, and in the case of Yuuta Yamauchi, a torrid love affair with your cousin. Summer Sis details the sexual relationship between Yuuta and his cousin, Kei Higashide, who offered to help him study for his college entrance exam. As their relationship develops, Kei becomes aware of a girl named Youko who may be toying with Yuuta. Yuuta's summer is about to get very complicated, and it's anybody's guess if he'll actually get any studying done!

Three boys: Yuuichi, Takidani and Abiko. Three girls: Nagisa (Yuuichi's little sister), Kanami and Youko. This is the story of complicated feelings and love triangles between these six students during their last year in high school, as they discuss and settle down about their future, specially love.