This is likely to be a short, I expect no great shakes from it, just a little fun, but if someone else wants to carry it further... We shall see what we shall see.
The basic outline is this: A horny elder sister has it "in" for her younger brother, intending to use him to fulfill her needs while she's between boyfriends. He isn't too keen on the idea, something about "incest taboos." Still, she is relatively attractive, and female, and he hasn't seen much in the way of action for his entire life, so she's starting to get to him. One day he gets home when she has an after school club meeting to attend, and finds himself relatively randy after being coyly teased by her for most of the day. He heads up to the bathroom to releive himself, but unbeknownst to him the meeting was cancelled, and she arrives shortly after he does, heading to the bathroom, who's door he had left a bit ajar, thinking the house would be empty for some time, and catches him masterbating. From there, I expect she'll use his hightly aroused state to get what she wants from him.
Now, those roles are set in stone, it could be an older brother persuing a younger sister, a mother persuing her son, a son persuing his mother, a brother in law persuing his brother's wife, etc. so long as there is some sort of strong taboo that makes one of them reluctant to participate. I am willing to play any part, but have a mild preferance for playing the victim. I am willing to run more than one version of this, so long as they're different enough. Any takers?