Otouto no Tomodachi no Erogaki ga Uzai!

A kind older sister finds herself the target of her younger brother's perverted friend, a shameless boy who constantly gropes her breasts. Despite his annoying behavior, she decides to take matters into her own hands and teach him about ejaculation.

However, once the boy experiences the pleasure of ejaculating on a woman's body, he becomes completely unrestrained. His newfound obsession pushes their relationship into increasingly explicit territory, as the older sister struggles to handle the consequences of her lesson.