Shujinkou-sama no Imouto ni Aisareteiru kedo, Shikkari Yandere.

Shinji Usui is a high school student whose childhood friend, Hinata Ichinose, is good-looking and hugely popular with girls, the sort of boy who could headline a romantic comedy. Standing next to someone so dazzling made Shinji feel small, and over time he grew awkward in Hinata's presence. Strangely, Hinata's younger sister Tsukina is warmer toward Shinji than she is toward her own brother. While Shinji is still wrestling with these tangled feelings, an unsettlingly worded love letter arrives, and the story starts there.