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

Male high school student Usui Shinji has a handsome childhood friend, Ichinose Hinata, who is as popular with girls as the protagonist of a story. Being beside Hinata, whose charm draws everyone in like the sun, always makes Shinji feel inferior, and before he knew it, he had started to dislike him a little. Meanwhile, for some reason, Ichinose Runa, Hinata's younger sister, is very attached to Shinji as if he were her real older brother. As Shinji goes about his days burdened by these complicated feelings, the story begins when he receives a suspicious love letter.