"Unfaithful Wank"

You've gone to your girlfriend's house, but when you make your way to her bedroom, your girlfriend is nowhere to be seen and you find her sister looking through her clothes. She's going on holiday soon and is rummaging through her sisters' clothes deciding what to borrow. But her nipple is hanging out of her top and it's making your cock rock hard. She can see she's accidentally turned you on, but thinks you should wank your cock and get it out of your system!