I love isolated, no boundaries wincest.
Dean who hit puberty way before Sam and just jerked off when he felt like it because there is no privacy in a motel room. Sam watching out of curiosity and asking questions, Dean learning that he doesn't just not care about the audience, he likes it.
Dean teaching Sam how to make himself feel good. Demonstrating his favourite techniques on himself and Sam copying. Dean teaching Sam how to stave off his orgasm until he's begging Dean to let him come. He never even told him he needed permission, that just developed organically, and it makes Dean lose his mind.
One of them had a wet dream so they lie practically on top of each other on the dry side of the bed.
Dean being a creep and watching porn in front of Sam, sometimes not even to jerk off, but just because he's bored. Watching porn together and jerking off side by side. It progresses to jerking each other off because it's no big deal, they're basically just one person at this point. What's the big deal about helping your brother out?
Brotherly bickering over who blows who first, once they cross that line. Dean always throws scissors and lets Sam decide anyway.
Competitions for who can make the other come first, using all the tricks they learnt on each other - they know what gets each other off.
They don't even see it the same way as sex with other people. It's just something they do. Dean, being older, understands how screwed up it is first but doesn't say anything to Sam; he doesn't want to stop.