Originally a custom! Your girlfriend's little sister has been staying with you for the weekend, and she discovers that you're turned on by her loud smelly burps. At first she's embarrassed "I can't help it!" But then she realizes you like smelling and hearing your girlfriend's little sister's burps. She giggles and teases you, agreeing to burp more and encouraging you to stroke your cock. She asks you what else turns you on... her dirty little bumhole? She slips a finger in and slides it in and out of her stinky asshole while you stroke yourself off, telling you she wants you to cum for her as she burps some more.