From Fan History Wiki
Incest or Incestfic refers to fanworks that feature a romantic or sexual relationship between two people who are closely related to one another. It is a term that means the same thing in real life as it does in fandom. Because of cultural taboos and laws, incest in fanworks has long been a subject of debate, disdain, and wank within in the media fandom community.
Incest stories and art are found in a variety of fandoms, including--but certainly not limited to--Angel Sanctuary, Digimon, Harry Potter, Chronicles of Narnia, Smallville and Ouran High School Host Club. Two of the most high-profile fandoms in recent years to feature prominent, popular incest pairings are Supernatural and Wizards of Waverly Place, where brother/brother, father/son and brother/sister incest is found.
What makes incest such a compelling kink in fandom is likely due to the same reasons why it is a kink in "mainstream" pornography--the taboo nature of the relationship, a situation that most would never want to embrace in real life but find compelling to explore in fiction. That said, many find the popularity in some circles of incest fic and art to be a sign of fandom's interest in pushing boundaries just to cause controversy and for shock value. It would appear that incest is a topic that fandom will continue to hotly debate for many years to come.
Below is a partial timeline of events in fandom that featured incest.
- Simon & Simon would be one of the earliest known, large-scale incest-fic fandoms in the 1980s and 1990s, although it was not widely embraced within the larger slash community.
- On July 31, 2004, the LiveJournal community heterocest was created. 
- In late February/ early March 2005, God Awful Fan Fiction posters went after Pornish Pixies for a Harry Potter story by ailei which amounted to a "chan/slash/incest/twincest porn story" because God Awful Fan Fiction posters did not like that minors were able to readily access this story. 
- INC—Incest. Surprisingly... or not surprisingly...is found often in C.S. Lewis fics and often in fics where the characters (siblings) are alienated from society. Ninja Turtles is a particular breeding ground. Could make an interesting Jerry Springer episode.
The following definition is drom Futeishi Anime Dicationary with an unknown date:
- (n.) Literary, film, or fan fiction term: A story that involves close family members falling in love (and possibly having sex).
- For example, some people in the Naruto fandom like the pairing of Neji and Hinata. Since they are first cousins, this is an incestuous relationship.
- In the anime Ouran High School Host Club, there are suggestions that twin brothers Hikaru and Kaoru have an incestuous relationship. (Often referred to as “twincest” in the fandom) 
Controversy in fandom
Incest fic is one of the most controversial topics in fan-fiction today, and has been such for many years. A post at tv.com addressing incestfic in Supernatural fandom has been hosting debate for almost three years as of June 2009, with over 24 pages of comments. To many, incest is one of the last, biggest taboos in fan-fiction, along with chan, shota, and for others real person fic.
That said, there are many who find incestfic to be a powerful kink, a dangerous topic made "safe" to explore through fictional constructs. Some authors, like Bhaalspawn, Material, and Loessar have said that incest fics make for the ultimate romance fiction if done properly, as a person's love for someone has to be strong to overpower the Westermark Effect. Bhaalspawn himself did an incest pairing in one of his fics, though it's used more as Sexual Tension.
Others will point out that the drive to write incest fic can be caused due to the intense chemistry of actors on screen that suggest that seems rife with subtext, and would clearly be read as sexual tension if not for the fact they are supposed to be playing related family members. Certainly Wincest is a case of this, in Supernatural fandom, as well as Jalex in the Wizards of Waverly Place fandom, a pairing that has its own page of notable subtext on the famous Tv Tropes site, and it was also pointed to as a reason for the popularity of the Lionel Luthor/Lex Luthor pairing in Smallville.
Popular Incest Pairings
- American Football: Peyton Manning/Eli Manning
- Family Guy: Stewie/OC Niece (Usually a daughter of Meg)
- Good Charlotte: Benji/Joel
- Harry Potter: Fred/George
- My Chemical Romance: Gerard/Mikey
- Naruto: Neji/Hinata 
- Ouran High School Host Club: Hikaru/Kaoru
- Smallville: Lionel/Lex
- Supernatural: Sam/Dean
- Wizards of Waverly Place: Justin/Alex
- Chronicles of Narnia: Peter/Susan, Edmund/Lucy and Peter/Edmund
The following terminology is related to incest, but specific to certain sub-genres and fandoms with a large enough incest-fic community to encourage its own specific name: