Who's Surprised?

When you burst into your sister's room you expect to find her making out with her boyfriend. But instead of being startled and embarrassed, Donna just stares at you and continues to slowly lick Brad's cock. You find you can't look away as you watch her pink tongue slide up and down the thick, hard shaft.

You don't realize that she's started grinning, now that it's become obvious, as you lick your lips, that it's not her breasts that you're focused on.