Sibling

A sibling is a brother or sister considered irrespective of sex.
Some fans add in extra sibling when writing fanfiction and for different reasons. For example, fan authors who want to write themselves into the story might give Harry Potter a brother or sister. Sometimes, these characters are Mary Sues. It's also common to write crossovers in which a character has a sibling from another fandom. 
Finally, a fan might take two unrelated characters from the same canon and make them siblings in fanfic.
==Examples of Siblings in Fanworks==
LilyK has a set of Sentinel zines called Bittersweet and Sacrifices and Miracles which focus on Dr. Blair and his sister, Dr. Brynn Sandburg.
Starsky & Hutch fanon has many fanfics that give Hutch a non-canonical sister, usually called Karen (Kathy?), a character who appeared in an unfilmed script. A list of Hutch's sister fics is at Tagging Bay City/Canon Characters' Fictional Extended Relatives
Siblings: An Illustrated Anthology is a multifandom fanart anthology zine in two volumes focusing on siblings from various canons. (2016, 2017)
==Secret Siblings==
A secret sibling is a brother or sister who appears, or seemingly appears, out of nowhere. This sibling may be known to the character, but not known to the audience. The secret sibling may be a secret even from the character, perhaps a child of a parent's previous relationship. Sometimes canon characters discover they are each other's siblings.
The secret sibling is usually a plot development used as backstory. Sometimes the secret sibling is an Evil Twin who creates conflict when people are initially unable to tell them apart.
==Secret Siblings in Canon==
Secret siblings, as well as other previously unmentioned relatives, are sometimes written into an ongoing serialized narrative long after the regular characters are established. Comics and soap operas are particularly prone to this trope. These characters may be derided as poorly executed retcons or embraced wholeheartedly by fandom.
*While technically not a secret as Starsky, and supposedly Hutch, know of Nick Starsky's existence, Starsky's brother isn't mentioned until the fourth season. 
*In The Sentinel, Blair is surprised by the existence of Jim's brother.
*In Stargate Atlantis, Rodney McKay doesn't mention that he has a sister until close to the end of the first season, and she doesn't appear in the flesh until several seasons later. Nonetheless, Jeannie Miller is a popular character.
*In the penultimate episode of Due South, Benton Fraser discovers that he has a half-sister. Some fanfiction ignores her existence, but other stories incorporate her as a sympathetic character.
*In Star Trek V, Spock reveals that he has a half-brother. Although the film is generally considered to be the worst Star Trek film ever made, the half-brother (and the implied previous marriage of Sarek) gets mentioned in both tie-in novels and fanfiction.
*In Supernatural, discovering the existence of (slightly undead) Adam Milligan, their younger Winchester half-brother, was a surprise to Sam and Dean Winchester in the Season 4 episode Jump the Shark.
*In Dexter an early canon Big Bad is Dexter's previously unknown brother.
*In NCIS season 9 Abby Sciuto volunteers to donate a kidney, learns that another volunteer has near-identical genetics, and discovers that he is her brother. She subsequently learns that her "little brother", mentioned in previous episodes, is not her genetic brother, and that neither of them were actually related to their parents; they were adopted separately, but their adoptive parents died before they were told. Abby tells the "real" brother about their relationship in a later episode, but this has had no long-term effects on the series to date.
*Dawn Summers, Buffy Summers' sister in Buffy. She is never mentioned until the start of Season 5; There are reasons.
*In World Wrestling Entertainment in 1997, Paul Bearer, the former manager of The Undertaker, claimed that Taker's family had "died" in a fire when Taker was a kid but changed his story to Taker's parents were dead but that Taker's brother Kane had survived and was coming back for revenge
*At the end of season two of Beforeigners, it is revealed that Alfhildr Enginnsdóttir and Lars Haaland are siblings
==Secret Siblings in Fanon==
* The Sentinel series Birthright by Rimilod is about Blair Sandburg and Jim Ellison discovering they are brothers
* In the Sentinel fic Hunted by Rimilod, Blair discovers Naomi Sandburg is not his mother but his sister
* Jim Ellison and Mack Wolfe (of One West Waikiki) who are both portrayed by actor Richard Burgi discover they are twins in Sugar is Sweet by Russet McMillan
* The Beauty and the Beast zine The Spiral Staircase features a story called "Happily Ever After." Vincent discovers he has a twin named David whose empathic senses, which are more intense than Vincent's, have turned into a debilitating curse that has exiled him from nearly all human contact
* In by Fyre, Willow Rosenberg, a main character and witch from Buffy the Vampire Slayer, turns out to be the long-lost twin of Ginny Weasley from Harry Potter
Category:Glossary