From ANN:

The story revolves around Tomoya Mizuhara, a 13-year-old boy who suddenly gains a big sister when his father remarries. 17-year-old Ichika is a little strange, and her affection for Tomoya is rather overwhelming, if not scary. On top of things, Ichika's friend Ruri is the ultimate sadist. Then there is Ichika's big-breasted quarter-Japanese friend Marina.