Aneki no Kounai Kaikinbi

When 23-year-old Hirose Anna finds out that her little brother masturbates with her dirty underwear, she decides to take things a step further by allowing him to see her do naughty things as well. Each time the situations escalate until she comes out and tells him that as his big sister she will take responsibility for his sexual curiosity. She allows him to do anything with her except for proper sex; yet as his requests get dirtier each time, she remains determined to take responsibility and keep things going no matter what. (Source: ANN, modified at BakaBT)