Incest

Incest is romantic and especially sexual goings-on between close relatives. It is strongly taboo in most cultures in the multiverse, and often illegal to boot. This does not stop fans, usually of the Movies, writing fanfic involving brothers or sisters having it off willingly, or parents abusing children. The PPC does not condone incest, and it is usually a charge. In cases where a Sue claims to be a sibling of a canon character, and yet still ends up having it off with them, this is still considered incest.

Incest is a common element of badfic because it is usually written in ways that force the characters to act extremely OOC, while ignoring the most basic principles of psychology. Due to the Westermarck effect, children who grow up living in very close proximity before the age of six are almost never sexually attracted to each other.

Depending on the culture, relations between cousins may or may not count. For example, it's extremely uncommon but still legal to marry one's first cousin in the United Kingdom and some US states. Relations between adopted siblings are also likely to fall under the heading of incest, unless they met when they were old enough for the Westermarck effect not to apply.

Relations between martial siblings (e.g. people raised in the same martial arts or cultivation sect) are not considered incest. Chinese culture commonly refers to people by familial terms regardless of actual kinship, which can lead to some confusion for people not aware of that.

Incest can still occur under more realistic conditions:
 * Siblings raised apart may be attracted to each other; for example, Luke and Leia of Star Wars, who were lucky to find out they were related before things went too far. Luke and Leia backed off pretty quickly once they found out, as most people would; but on occasion, characters who find out they're siblings may still find themselves attracted nevertheless. How they react to this depends on the character; and if their reaction is not in character, incest is still a charge.
 * Incest may take place in sexually abusive and/or very dysfunctional families. Note that making a family dysfunctional just to allow for incest is still a charge if it pulls characters out of character.
 * Non-human or very unusual cultures may not have an incest taboo; for example, the royal families of ancient Egypt often married brother to sister (it helped that they were raised apart). In A Song of Ice and Fire, the Targaryen Dynasty does the same. If the writer simply states that the incest taboo is not present, with the sole purpose of writing incest!fic, it's still a charge.

Portrayal of Incest in Fanfiction

 * Twincest is a specialized kind of incest that takes place between twins. The most notorious victims are Fred and George Weasley; Dipper and Mabel Pines; and Elladan and Elrohir Peredhil.
 * There are many documented cases of Self!cest, which are usually accomplished in sci-fi continua where there had been several instances/methods of duplication of the canon characters. Some common stories are between William T. Riker and his transporter clone, Thomas; or Hermione while using the Time-Turner.
 * Characters particularly vulnerable to incest tend to be related main characters who are both attractive. Believing that romantic relationships are more "real" than sibling relationships, the badficcer forces the characters to commit incest. This is common in the Loud House fandom, where Lincoln Loud is paired with any of his 10 sisters. See Wincest.
 * Incest, used in a more realistic, abusive way, can be a part of bashing a character. If the character would never commit incest, it is OOC and a charge.

Cautions for Exorcists
Characters who have just been exorcised after an incest!fic are best neuralyzed immediately. Depending on the character, they are likely to vomit, attempt to kill themselves, suffer catatonic breakdowns, or attack whoever happens to be closest. If neuralyzation is impossible, rendering the character unconscious is a reasonable alternative. FicPsych can handle cases where neuralyzation is not effective.

Missions Involving Incest

 * "To Know Where You Are Going" (Lord of the Rings), Agents Jay and Acacia (DMS) with Agent Luxury (DBS)
 * Elrond and Arwen: the love story never told. For a bloody good reason, as Jay and Acacia... explain to the Possesion-Sue.
 * Sue: Sue!Arwen