Aneki no Kounai Kaikinbi

When 23 year old Hirose Anna finds out that her little brother masturbates to her dirty underwear, she decides to be a step ahead of him: she chooses to allow him to see her naughty side, and escalates her actions each time.

Eventually, she comes out and tells him she will take care of his sexual curiosity as his older sister: she will allow him to do anything, except vaginal intercourse, and while his requests get dirtier each time, she takes responsibility as his older sister, stands by her word, and keeps going.