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

Usui Shinji, an ordinary male high school student, has spent his life beside his childhood friend Ichinose Hinata—a strikingly handsome young man whose popularity with girls rivals that of any story's protagonist. Standing next to Hinata, whose radiant charm draws everyone to him the way the sun commands the sky, Shinji cannot shake a persistent sense of inferiority, and somewhere along the way, that quiet inadequacy hardened into a slight dislike he never expected to feel toward his own childhood friend. Complicating matters further is Hinata's younger sister, Ichinose Runa, who, for reasons Shinji cannot quite explain, has grown deeply attached to him—treating him with such complete devotion that he might as well be her real older brother. Her unwavering affection adds yet another layer of awkwardness to his already burdened days, leaving Shinji to trudge through school weighed down by a tangle of emotions he can neither untangle nor ignore. Then everything changes when Shinji receives a suspicious love letter—a single, unsettling note that sets the entire story in motion and threatens to upend his complicated life.