A common sense-altering AV with the restriction that it's OK if the requests are of equal value! If the scales of requests are balanced, the other person can never resist! Do whatever you want with your sister or her friends in exchange for something erotic! If you lick her hard, you'll get a hard blowjob in return! The forbidden, absurd, equal exchange is established!? Your sister's friend becomes a bitch! You finish with a massive creampie deep in her vagina!