So they get to this party, with Sam looking very much like a girl in the clothes his friend made him wear, and Dean almost immediately takes an interest in 'Her'. He spends most of the party seducing and taking care of Sam, supposedly unaware that she's actually his little brother, and Sam doesn't say a word to Dean which he takes to mean she's shy, but he does it because he knows Dean will recognize him.

Eventually, Dean succeeds in seducing Sam and they have sex. When Dean finds out she has a dick he just seems to think it's even hotter. Afterwards, Sam's kinda mortified because his friend he was there with knows that Dean's his brother and knows that he had sex with Dean.

Sam gets home first and changes back into his regular clothe and stuff and when Dean gets home, he kinda mentions that he knew who Sam was as soon as he saw his eyes. I'm pretty sure that was the end of the fic, but for some reason I'm also thinking there's a sequel, as well.