Brother and sister make bet. Brother asks for "slave" or something similar as the penalty and she is so sure that she agrees. Brother pushes the sexual envelope during the period and sister reluctantly complies. Brother starts suspecting that sister enjoys the situation and is only playing a part after she loses another bet to extend the penalty.
Details I'm not completely sure about: bets are made on "who plays Morpheus in the Matrix" and on Mario Kart. There is a cousin/neighbor and her brother who get involved.
My search-fu is weak or there are simply too many stories with similar themes. I'm a little scared that the story is no longer available.
Thanks in advance.
