Dean really needed to work on his signals. He was giving off some very mixed ones right now. His words were saying 'this is wrong, we're brothers, stop', his damn mouth was saying 'kiss me now' and certain other parts were poking Sam and saying 'take me now, I'm yours'. Mixed signals. Lots of them.
The logical part of Sam's mind told him to pull away. They could blame it on the alcohol and the emotions of the evening and could move on and forget about it. But there was something about the way Dean looked at him that made his heart clenched and his mouth go dry.
And so he risked it again, leaning in to press his lips to Dean's, firmer this time.