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

High school boy Shinji Usui has a childhood friend named Hinata Ichinose — a guy who’s handsome and incredibly popular with girls, just like the hero of a rom-com. Being around Hinata, who’s as radiant and captivating as the sun, made Shinji feel pitiful by comparison, so at some point, he began to feel a bit uncomfortable around him. On the other hand, Hinata’s younger sister, Tsukina, treats him more kindly than she treats her own brother, for some reason. The story begins when Shinji — who has been trying to come to terms with these complex feelings — receives an ominously worded love letter. (Source: Natalie, translated)