Better Days and Badly Drawn Kitties, which had a shared cast of characters, including twins Fisk and Lucy Black. The two comics used the sibling's special relationship in completely different ways. Better Days used the relationship dramatically, while BDK used it as a Crosses the Line TwiceRunning Gag.
Although as Better Days progressed both characters seem to deny it happened, it slowly became a forgotten dropped storyline, and both moved on to their respective Relationship Sue
Powerpuff Girls Doujinshi takes the mild vibe between Dexter and Dee Dee mentioned below and runs with it.
Concession has Rick and Kate (who are also a womanizing drug dealer and a pedophile respectively), plus Joel and his dead sister Miranda, possibly, she died as a young child but is really close to her brother as a ghost
In Juvenile Diversion, the reveal that Julie is Jason's half-sister freaks him out, due to his feelings for her (and the fact that she slept with him).
Absolute Hot Sister is built on this trope. Ellen has gone so far as to suggest to her brother Kevin that they sleep in the same bed and take showers together.
ThisStuds Up comic has two people discussing this trope as an analogy to being a Spurs fan. They almost got to the logical conclusion before Squick sets in.
