The catch is that they don't find out that they're sisters until after they've fooled around.  :-)  A little extra melodrama, y'see.  This isn't just two hawt and sexy babes having fun...it's two lonely and conflicted souls finding comfort in each other after resisting temptation for as long as possible, then discovering that they've just broken a major taboo.  (shock!  Horror!)<br/><br/>Not my normal thing, but neither was Curse of the Golden Flower, which has a body count of Elizabethan Tragedy proportions (and also stars Chow Yun Fat playing a real jerk of an Emperor).