"Sleepover Babe"

Your sister is having a sleepover tonight, so you're staying out of her way by sticking in the lounge. One of the girls walks in to read a book but is unaware you're there. As she's reading, you can see her nipple has fallen out of her tiny top. She finally spots you, and explains all the girls had fallen asleep, so that's why she's downstairs reading a book, but then she realizes her nipple is exposed and you've been perving the whole time!