Dean has always enjoyed sex. A lot. Maybe a little too much. When Sam is 15 and Dean is 19, John leaves them in a cabin up in the middle of nowhere for several months. No contact with the outside world besides an old friend of their father who drops off supplies for them every few weeks then leaves. As the weeks pass, Dean grows increasingly restless, until finally after one too many drinks he has sex with his little brother. Of course he's horrified and ashamed with himself. Only he didn't count on Sam actually liking it, and doing everything he can to make his brother do it again.
The premise of this RP is that the first time Dean has sex with Sam it is very dubious consent, almost bordering on abuse. Sam feels betrayed and hurt by his brother's actions at first, but eventually comes to terms with the fact that he actually liked it and would like to do it again. Still, he thinks that Dean needs to be taught a lesson and doesn't do anything to aleviate Dean's intense guilt over what he's done. Instead sets out to 'punish' his brother by doing all kinds of things to make Dean 'crack' again, like walking around sans clothing, then making Dean feel even more guilty over it.
This tale could be fairly humorous, or it could be very angsty. Especially as Dean's guilt over what he's done grows. Or maybe a little of both. Starting out humorous and getting angstier as Sam doesn't fully understand what he's doing to his brother. Could go several ways.