Say the girl spends her vacation with her aunt and uncle who has a private cabin at the lake. The first night there she can't help over hearing them having sex most of the night. The next morning her aunt and uncle act normal. After sunbathing, she comes into the cabin and hears them going at it again. The door to their room is half open and her curiousity gets the best of her. She watches as her uncle fucks her aunt like she wish she could be fucked. They notice her but pretends not to. Later that night she hears them going at it again, but this time in the main part of the cabin. She watches from her bedroom door fingering herself imagining that it was her taking her uncle's cock. Again, her aunt and uncle notice her, but pretend not to. The next day, down by the lake, her aunt conviences her to go skinny dipping with her after a pitcher of (you pick it). The uncle shows up later and the aunt brags about how beautiful and sexy the girl's body is and urges the uncle to feel hers and the girl's breasts.
That should be enough to get things started for your story.
