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

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

(Source: Natalie.mu, translated)