I'd like a scene where the sister can see her brother wanking in his room and at first is a little disgusted but then when she feels she is alone and not likely to be disturbed gets a little turned on and starts to play. Then, just as she's enjoying herself she knocks something over and attracts her brothers attention leaving her to be a litlle embarrassed and trying to give an excuse as to why she looks so flushed!! Just a thought, I know the idea of my sister watching me makes me horny. 
