Onee-chan ga Kita

Tomoya Mizuhara, a 13-year-old boy, finds himself with a new older sister after his father remarries. His step-sister, Ichika, is 17 and displays an intense, unsettling level of affection toward him. Adding to the complexity is Ruri, Ichika's friend known for her sadistic tendencies, and Marina, Ichika's half-Japanese companion who is described as having a large bust.