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
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.
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
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.
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.
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.