Crimson Echoes
Read the storyItadori has being touching himself to nsfw audios of a creators name bloodymess, they are mostly audios of the creator rolplaying that they are an older sister in sexual matters(like finding her brother masturbating or things like that), he really likes them because they remember him to his own sister:Choso. One day he wakes up and hears his sister making noise in her bedroom and finds her making the audios... Lets just say that the next day bloodymess loaded a video name 'Younger brother finds sister touching herself