In this trope, a third person is introduced specifically in a way to complicate the relationship between established siblings, or between the established siblings and the new sibling. If the characters aren't related by blood, but through the same MetaOrigin consider themselves BloodBrothers or BashBrothers ''literally'', then another character with the same MetaOrigin and honorary sibling status may suffice.

Consider the trope namers. Many people know the story of CainAndAbel, the first children born in the Abrahamic religions. AsYouKnow, Cain kills Abel, and was banished by God. What fewer people know, is that AdamAndOrEve had several more kids, one of whom was named Seth. Seth would replace Abel as the "good" son, and while both he and Cain would have children [[BrotherSisterIncest (with their unnamed sisters)]], Cain's lineage would be cursed, and would eventually all be doomed by the Great Flood. Noah and his family were descended from Seth, and as the only survivors of the Flood, this means that all humans are descended from Seth, taking both of their places as the future of humanity. Note that despite "Seth" being much more common as a given name than "[[NameOfCain Cain]]" or "Abel", this is unrelated to AerithAndBob.

Considering these characters can be introduced quite some time after the original set, spoilers may be ahead.

Compare LongLostRelative, which can possibly be a supertrope to this, CousinOliver, RememberTheNewGuy and TangledFamilyTree.
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!!Examples:

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[[folder:Anime & Manga]]
* The late introduction of Shizuku in ''CandyBoy'', with her BigSisterWorship of Yukino and slight SiblingRivalry with Kanade, really shakes up the twins' dynamic.
* In ''Manga/OnePiece'', Luffy and Ace are revealed [[spoiler:to not be related, and have a third [[BloodBrothers Blood Brother]] named Sabo. In their youth, he was the physical midpoint of the three, and was TheSpock in their three man band. The story is told entirely through flashback, as Sabo is assumed dead and thus a PosthumousCharacter. His death, and Ace's subsequent death]] complicates every ''previous'' interaction between Luffy and Ace, their current situation, and [[GallowsHumor all those that would follow]]. Additionally, Ace's tattoo and Luffy and Ace's relationships with their TrueCompanions take new significance. In a cover, it is shown that [[spoiler:had Sabo lived, the three would be together, and to cement this "Good Future", Ace's tattoo loses its meaningful misspelling]].
** [[spoiler:It becomes further complicated when it's revealed that Sabo ''survived'' the incident that supposedly took his life, and has been in hiding for the last twelve years, apparently as a member of the Revolutionary Army. While undoubtedly a heartwarming revelation, it becomes HarsherInHindsight when one realizes that Ace died without ever knowing that his other brother was alive.]]
* ''Anime/YuGiOh'': In the anime, the Virtual World FillerArc stars Noah Kaiba, the previously-unknown biological son of Seto and Mokuba's adoptive father. He has an enormous grudge against Seto and wants to take away everything he has-- including Mokuba, who he kidnaps and brainwashes into thinking Noah is his only brother. The brainwashing doesn't hold, but after Noah has a HeelFaceTurn Mokuba says that he's family.
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[[folder:Comics]]
* In ''ComicBook/{{X-Men}}'', a slip up by Mr. Sinister introduced an example. As explained on the TangledFamilyTree page, Scott Summers was introduced as an orphan, then met and established a relationship with his brother Alex. This lasted for quite some time, before Sinister hinted that there was at least one more brother out there. After a lot of plot changes and WordOfGod statements, the brother was set as Vulcan, a supervillain emperor. But of course, given that Sinister never gave a specific number, any or all of the other implied candidates could still ''also'' be Summers brothers.
* Some continuities of ''TheMightyThor'' include Balder as the Seth to Thor's Abel and Loki's Cain, though as far as their father Odin's concerned, Balder is the only "Abel" there, with both Loki and Thor viewed as reckless and wayward sons.
** Marvel's upcoming ''Original Sin'' storyline promises to complicate this still further by introducing a previously unknown ''sister''. [[spoiler:It's Angela, originally a supporting character in ''{{Spawn}}'' and more recently a regular in ''GuardiansOfTheGalaxy''.]]
* Damian Wayne, the latest Robin, is this in spades. He's the youngest Robin ever, has a massively abrasive personality and is the only Robin to actually be a blood relative of {{Batman}} (much to the discomfort of Dick Grayson and Tim Drake, who are adopted sons of Batman).
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[[folder:Literature]]
* The Noldor royal family in Creator/JRRTolkien's TheSilmarillion: Fanor (Cain), Fingolfin (Abel) and Finarfin (Seth). [[spoiler:The only surviving Noldor royal family members after the First Age were Finarfin, his daughter Galadriel and Gil-Galad, an elven prince of undecided heritage. Arwen was Galadriel's granddaughter and Finarfin's great-granddaughter. There was also Maglor the Feanorian, who turned into the Elven version of the Wandering Jew and [[PutOnABus never entered the story again]].]] Tolkien never finally decided whether King Gil-Galad was a descendant of Fingolfin or Finarfin; in the published Silm he's the former, but in Tolkien's latest drafts he's the latter.
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[[folder:Live Action TV]]
* In ''Series/{{Supernatural}}'', Sam and Dean have spent most of their twenty-some years with only each other as dependable family. Eventually, in the episode entitled ''"JumpingTheShark"'' including references to [[TheBradyBunch Cousin Oliver]], they find out that their father had a child several years after their mother died, whom he kept from the hunter's life to protect him. The brothers immediately argue about whether to train him or to spare him from their life. It turns out [[spoiler:he's already been killed in revenge against their father, and they've been dealing with a ghoul]].
** When Adam eventually makes another appearance in season five [[spoiler: as himself, after he's been [[BackFromTheDead brought]] back [[BackForTheDead from the dead]] as bait or a [[TakeAThirdOption possible alternative]] for Michael,]] he doesn't get along with either of the brothers that never knew about him. In Season six, when Dean's given the option to either [[spoiler:save Sam's soul from eternal torment in Lucifer's cage or rescue Adam's from the same place, Dean [[BigBrotherInstinct unsurprisingly]] doesn't even hesitate in choosing [[{{Protectorate}} Sam]]]].
** Simultaneously, within the archangels, [[ArchangelLucifer Lucifer]] and [[ArchangelMichael Michael]] are seeking the same Armageddon, each thinking they would win. Winchester brother intervention gets [[ArchangelGabriel Gabriel]] involved, introducing this dynamic between them to the viewers. Gabriel takes neither side, choosing ''humans'' instead, and is responsible for [[spoiler: telling Sam and Dean how to seal the both of them in Lucifer's Cage, preventing the conflict that would raze the world]].
* ''Series/{{Charmed}}'' revolved around three sisters, the [[TitleDrop Charmed Ones]]. One of the sisters died early in the show, so a half-sister was added down the line to complete the trio. The two elder sisters (Piper and Phoebe) tried to enlist the help of the new sister (Paige), who was reluctant to join them and almost joined the {{Dark Side}}. There were also episodes which focused on how Paige's personality affected the dynamics within the family (namely, an episode where Paige tries to rearrange everything in the house which causes tension between her and Piper). Paige was also far more emotional on decisions concerning the past of their grandparents.
* The show ''{{Sisters}}'' originally had 4 sisters: Alex, Teddy, Georgie and Frankie. In the 4th season they discover they have a younger half-sister named Charley who was born from an affair their father had. Charley was a guest star in seasons 4 and 5 but when the actress who played Frankie left the show Charley (now with a different actress) became a regular character during the sixth season.
* Venus de Milo of the short lived ''Series/NinjaTurtlesTheNextMutation'' series was a female turtle with the same MetaOrigin. But she got so much hate from the [[FanonDiscontinuity fans]] ''[[DisownedAdaptation and creators of the original series]]'', and the show got terrible ratings and was cancelled, so the whole thing is [[CanonDiscontinuity noncanonical]] now.
* ''{{Gossip Girl}}'', where Chuck becomes Serena and Eric's adoptive brother when their parents are married. The siblings already knew Chuck, and because of his earlier attempt to rape Serena, neither of them trust him. While Eric is the first to come closer to Chuck, Serena still distrusts him and blames him for sending her porn magazines. [[spoiler: Though, as it turns out, they were sent by someone else. Their relationship gets even worse when Chuck's father dies]].
* While a member of the ''Series/{{Dallas}}'' cast from day one, it was eventually revealed that ranch foreman, Ray Krebbs, was Jock Ewing's illegitimate son. Naturally, Bobby accepts his new brother, while J.R. worries about another claimant to Ewing Oil.
* The relationship between half-siblings Arthur and Morgana on ''Series/{{Merlin}}'' is changed forever when Morgana's older half-sister Morgause (no relation to Arthur) appears on the show and eventually spirits her away. Interestingly, TheReveal as to Morgana and Morgause's relationship (maternal half-sisters) is discovered long before Arthur and Morgana (paternal half-siblings) are discovered to be related.
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[[folder:Video Games]]
* ''[[VideoGame/FirstEncounterAssaultRecon F.E.A.R.3's]]'' [[spoiler: Third Child]] is hinted in the good ending to have a brighter future than [[spoiler: its brothers.]]
* In ''VideoGame/MortalKombatArmageddon'' Rain's Ending [[spoiler: it is revealed he is the third son of Argus - the other two being rivals Dagon and Taven]]
* ''VideoGame/TeamFortress2's'' Gray Mann not only twists the family dynamic like Seth, [[spoiler: He one-ups Cain, and murders ''both'' his brothers. ]]
* ''[[VideoGame/FireEmblemJugdral Fire Emblem: Genealogy of the Holy War]]'' ends up with a show down between [[spoiler: protagonist Seliph and his half brother and half sister Julius and Julia. Depending on the players actions Julia can either be an enemy or ally.]]
** Though in all honesty, [[spoiler: Seliph inverts this, as Seliph is both the Seth and the oldest, and is also their half-sibling instead of full. Making Julia the Abel.]]
* In ''VideoGame/MetalGearSolid'', established character Solid Snake meets his twin clone brother, Liquid Snake. Designate Cain or Abel to whichever one you want (Liquid claimed that one of them was the brother of light, and one is the brother of dark, but he didn't care to specify who was who) however, Snake was the one who survived and Liquid was killed by [[spoiler: the FOXDIE disease carried by Snake.]] However, [[spoiler: as TheStinger in ''VideoGame/MetalGearSolid'' alluded to, there was a third "Snake brother", Solidus Snake. He shows up in ''VideoGame/MetalGearSolid2SonsOfLiberty'' as Raiden's primary antagonist, not Snake's.]] Interestingly, while the three brothers find themselves on opposing sides, they are all generally considered to be [[WellIntentionedExtremist Well-Intentioned Extremists]].
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[[folder:Webcomics]]
* ''Webcomic/TheOrderOfTheStick'' commits a LampshadeHanging in order to state that no, Elan and his evil twin Nale probably won't get a neutral brother named Lean or [[HehHehYouSaidX Anel]] or something.
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[[folder:Western Animation]]
* In ''WesternAnimation/TheVentureBrothers'', [[spoiler:Dermott Fictel is introduced, and almost instantly separates Dean and Hank, as he is more similar to Hank, following the brothers' DivergentCharacterEvolution. He's initially made out to be Brock Samson's son, but he is actually Rusty's accidental child, thus making him the third Venture brother, and strangely enough the only brother with a confirmed mother]].
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