She is 'Kimmie's little sister' and Kimmie is off to college. Her aunt tells her that her cousin is coming to spend the summer. He is given Kimmie's room and he finds Muffy's diary and we're shown what it contains via flashbacks. In the end, they all have an incest party.

She is Kimmies little sister and Kimmie is off to college. Her aunt tells her that her cousin is coming to spend the summer. He is given Kimmies room and he finds Muffys diary and were shown what it contains via flashbacks.