Onee-chan ga Kita

Tomoya Mizuhara is a 13-year-old boy whose life changes when his father remarries, giving him a new older sister named Ichika, who is 17. Ichika displays an intense and unsettling level of affection toward him. Adding to the situation is Ruri, Ichika's friend known for her sadistic tendencies, and Marina, Ichika's half-Japanese companion with a more prominent physical feature.