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.
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 an off-color Running 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 partners.
While BDK has rebooted entirely multiple times, removing Fisk.
Another of Jay Naylor's characters, Lynne, who has appeared a few times in Original Life, frequently screws her younger brother Genwyn. Sometimes against his will, though he initiates it at least once in their porn comic. And while Lynne gives a lot of guys head Gen is one of the few who've had intercourse with her.
This is a key part for one of Butterfly's slaves in Collar 6. She and her sister were both married to the same man, since polygamy is okie-dokie in this world, and he thought they should engage in this. She didn't agree while her sister did. That got her shipped off to attempt unpleasant "changing" in her thinking.
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, you could say possessive, to her brother as a ghost
Apple and Orange from Fruit Incest make no efforts to hide their relationship from the audience, to the point that their cousins are more visibly squicked by their Sickeningly Sweethearts status than the actual incest part.
Touched on in Girl Genius — Word of God says that Gil and Zeetha are fraternal twins separated at a young age — which makes the scene where Gil walks around the room naked with Zeetha watching appreciatively, until she points out that he's not wearing any pants, actually kind of squicky.
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, and might have had sex with him if not for the third wheel).
The The Loud House Spanish web comic Cueva del Caos is built around this trope, with Lincoln and Lynn getting into a relationship after coming across a secret room in their basement and Lincoln having affairs with Luna and Leni.
Rice Boy has a rather... uncomfortable sequence where Golgo, increasingly unstable and depressed, has sex with his apparent sister with their facial expressions implying that this isn't the first time. And unlike some examples, he really doesn't seem to enjoy it. Subverted as we later learn Golgo is a Searcher, meaning that she isn't really his sister at least not in the conventional sense (if she was she'd be over 3,000 years old by the time of the story). Somehow this reveal doesn't make the above scene any less unsettling.
The interactions between masturbation addicts Mike and Michelle in Sexy Losers are definitely in this vein, and like everything else in the comic it's supposed to be disturbing. Mike has peeped on Michelle in the shower, made a DIY dildo based on his own dick to give to her (which she wanted to use right away), and asked her to "take over" when he needed his hands free for something else.
At one point, Doctor Lovetalk's sex-therapy radio show takes back-to-back calls from a brother-sister pair trying to enact their violent rape fantasies on each other. She hangs up on both calls.
In Shortpacked! it's revealed that Amber and Faz are half siblings...which is a problem for Amber, as Faz had been hitting on her since the day they met. She's also reluctant to tell him the truth because she fears that that would just encourage him by invoking the genetic sexual attraction concept. She eventually tells him when their father dies. He does end up losing interest in her, though it seems to be less about them being siblings and more about him feeling betrayed that she didn't tell him sooner. (Him finding a girlfriend certainly helped, though.)
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.