Netorare Sister - Kasumi Haruno

This is the story of Ayato Haruno, a young boy living with his cousin, Kasumi, while his parents are overseas. Ayato is madly in love with the sexy Kasumi, and can't wait to go to the same high school as her.

One day he's masturbating to a video he finds online, and realizes that the girl in the video resembles his cousin, Kasumi. He then finds out that the girl in the video doesn't just look like Kasumi, it is her. Apparently Ayato's sexy cousin has a secret life when she goes out. Things spiral out of control, as Kasumi gets addicted to the new sexual world she can't say no to...

(Source: J-List)