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The story "Becka the Beast" (NSFW) is about Gia Cameron, a karate black-belt who falls in love with gay school bully Becka Jackson. It's quickly revealed that Becka is the victim of Parental Incest, coupled with a relationship with a Depraved Bisexual that has left her an empty shell. Gia's father adopts Becka in the end.
This has cropped up in Survival of the Fittest a couple of times (although in one case this was Twincest). Matthias Kovalenko of V2 was scarily obsessed with his sister Jodeen. (fortunately, she never made it onto the island). Nothing explicit ever occurs, but from Matthias' thoughts, most get the impression it isn't just brotherly concern.
Fortunately averted in Broken Saints: Shandala and Gabriel look like they might be bonding romantically prior to Tui Jr's death, but it is cut mercifully short. This is especially creepy when you realize Gabriel knows Shandala is his sister.
In the Chakona Space setting, there are a number of examples of this (as well as some Parental Incest) between members of various genetically-engineered species such as chakats and foxtaurs. Hermaphrodite chakat sisters Forestwalker and Goldfur enjoy recreational sex, and foxtaur brother and sister Garrek and Malena are also mated (this is hand waved with the explanation that they have been genetically engineered to eliminate the sorts of diseases and conditions that inbreeding can produce in humans, and as created species do not share the exact same moral squick factors as their creators).
Granted, Chakats and most other species still have taboos against inbreeding so Goldfur and Forestwalker are careful not to have sex when either one is in heat. Foxtaurs allow one generation of inbreeding because most of them can't leave their isolated Amish-style villages thanks to Territorial Attachment Syndrome, and a shortage of males.
Simon Bar Sinister and Penny Dreadful were a brother-sister team of supervillains in the Global Guardians PBEM Universe. In addition to being murderous psychopaths, they were utterly and completely in love with one another, to the point of getting physical while they were in the middle of a crime.
In Tea, Biscuits and Incest Mikayla eventually learns that her boyfriend Jayden is also her half-brother.
In supernatural web soap California Heaven , an episode features brother and sister, Dominic and KC, in a very seductive dance nearly kissing. Thought, it looks like KC was possessed. However, in a most recent episode, Dominic stares at KC as she is sleeping and she seems to have no problem with it.
Implied in Spooning With Spoony III, when Spoony fucks both The Nostalgia Critic and The Other Guy (along with plenty of other people) and tells the latter he makes noises just like his brother.
This was to have been a plot point in The Nostalgia Chick's video "Thanks For The Feedback" detailing a date between Chick and the Critic. In the original draft, they would have slept together and then been informed that they were actually brother and sister. This idea was scrapped prior to filming.
In The Gamers Alliance, Desdemona and Mordecai are siblings and very much in love although they have to hide their relationship from everyone else. Desdemona eventually gets pregnant with Mordecai's child and intends to give birth to the child, using it as a means to install herself on the Remonian throne by claiming that the child belongs to King Marcus.
Present in a benign form in regards to Ben in An Ordinary Sex Life. Adrienne on the other hand...
In Comix from the Underground, the Comix Scrutinizer suggests that the furry webcomic characters Kit n Kay Boodle are participating in this since they're not married and have the same last name.