A vaguely can recall this doujin I found back then when Hbrowse was still alive and I still couldn’t find it anywhere else. I remember it starts out with a boy getting bullied by another guy, then his older sister(forgot if blood related or very close childhood friends for him to call her that) in a school uniform saves him, I think I remember her smacking the bully too. Later on she ended up doing it with the bully, I vaguely remembered him kinda blackmailing her to do it with him so he won’t go after her younger brother anymore and the older sister eventually just likes doing it with the bully at the end.