Wait for her

You decide to give her a bit of privacy and wait for her to come out.

You go to her table and start up her Mac to check her internet history, curious about the authenticity of your sister's supposed innocence. Disappointingly, after several minutes of searching, you don't find anything out of the ordinary. The only pictures and videos she have are of really, really cute cats. Not that you don't have any pics of your own. Either she really is as innocent as she looks, or she simply knows how to go incognito.

Just then, you hear the shower get turned off. Scrambling, you close all windows on her Mac, shut down her computer, and go stand near the bed, waiting for her to come out. You hear her start humming in her sweet little girl voice. A couple of minutes later, the door opens, and out your sister comes with nothing but a white towel wrapped around her body (unfortunately covering the parts you most want to see). You look at her stunning body in awe. Her spotless, wet skin and beautiful, freshly-washed body just further enhances your lust. She shrieks when she sees you standing there looking at her, and she grips her towel tighter. "What're you doing here?!" she asks in a cute and high-pitched voice.