This is just an idea that's been percolating in my head for a while, and is driving me nuts. A brother and sister have a lot of time alone together, and being teenagers (at least 16 years old), they're fairly horny a lot of the time.
At some point one of them walks in on the other masturbating, and after the initial embarrassment, they're interested in seeing how the other gender does it, so they masturbate themselves in front of one another. This would escalate to masturbating each other, oral, and anal. Figuring that it's not REALLY sex, so it's not REALLY incest if they don't cross the final line of vaginal sex.
I could definitely see some emotional turmoil with this in addition to the sex scenes. And if/when that final line is crossed, how far are they willing to go?
If you're interested, drop me a PM. Remember, as long as you're capable of playing a female, I don't care what gender you are IRL.