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 Meta Origin consider themselves Blood Brothers or Bash Brothers literally, then another character with the same Meta Origin and honorary sibling status may suffice.
Consider the trope namers. Many people know the story of Cain and Abel, the first children born in the Abrahamic religions. In the story, Cain kills Abel and was banished by God. What fewer people know is that Adam and/or Eve 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 (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.
Considering these characters can be introduced quite some time after the original set, spoilers may be ahead.
Compare Long-Lost Relative, which can possibly be a supertrope to this, Cousin Oliver, Remember the New Guy? and Tangled Family Tree. Note that despite "Seth" being much more common as a given name than "Cain" or "Abel", this is unrelated to Aerith and Bob.
- The late introduction of Shizuku in Candy Boy, with her Big Sister Worship of Yukino and slight Sibling Rivalry with Kanade, really shakes up the twins' dynamic.
- In One Piece, Luffy and Ace are revealed to not be related, and have a third Blood Brother named Sabo. In their youth, he was the physical midpoint of the three, and was The Spock in their three man band. The story is told entirely through flashback, as Sabo is assumed dead and thus a Posthumous Character. His death, and Ace's subsequent death complicates every previous interaction between Luffy and Ace, their current situation, and all those that would follow. Additionally, Ace's tattoo and Luffy and Ace's relationships with their True Companions take new significance. In a cover, it is shown that had Sabo lived, the three would be together, and to cement this "Good Future", Ace's tattoo loses its meaningful misspelling. 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 Harsher in Hindsight when one realizes that Ace died without ever knowing that his other brother was alive. And even moreso because Sabo had amnesia the whole the time, only remembering his life with Ace and Luffy upon learning of Ace's death...at which point he collapses in tears because he wasn't there to even attempt to save his brother.
- Yu-Gi-Oh!: In the anime, the Virtual World Filler Arc 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 Heel–Face Turn Mokuba says that he's family.
- Played about as straight as it can be with Cain (the Big Bad), Abel (The Hero), and (The High Queen) Seth from Trinity Blood.
- Date A Live has Shido and Kotori (the former being adopted into the latter's family) suddenly encounter the former's younger biological sister Mana one day. What makes this complicated is that both Shido and Mana have amnesia and don't remember their pasts. Mana does have a locket containing a picture of her and Shido when they were a younger, which is how she recognises him when she coincidentally sees him in a video. The relationship is confirmed by DNA testing. The drama comes from the fact that Kotori is part of the organisation Ratatoskr, which seeks to control Spirits by making use of Shido's ability to seal their powers, while Mana is part of DEM, which tries to kill them, and she doesn't appreciate Kotori putting Shido at risk.
- In X-Men, a slip up by Mr. Sinister introduced an example. As explained on the Tangled Family Tree 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 Word of God statements, the brother was set as Vulcan, a supervillain emperor. But of course, given that Sinister never gave a specific number (he merely said "brothers"), any or all of the other implied candidates could still also be Summers brothers.
- Some continuities of The Mighty Thor 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 (usually).
- 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).
- Thor: Ragnarok: With Loki and Thor as Cain and Abel respectively, in comes Hela revealed as their older sister. The two end up working together to stop her.
- The Noldor royal family in J. R. R. Tolkien's The Silmarillion: Fëanor (Cain), Fingolfin (Abel) and Finarfin (Seth). The only surviving Noldor royal family members in the male line after the First Age were Finarfin, his daughter Galadriel, and Gil-Galad, an elven prince of undecided heritage. Arwen was Galadriel's granddaughter on her mother's side and Finarfin's great-granddaughter. There was also Maglor the Fëanorian, who turned into the Elven version of the Wandering Jew and never entered the story again. The House of Fingolfin survived in the female line, however, with Idril Celebrindal, daughter of Fingolfin's second son Turgon, marrying Tuor, a prince of Men, and giving birth to Eärendil, father of Elrond (whose significance should be obvious) and Elros (and thus ancestor to the royal family of Númenor and therefore of Arnor and Gondor. Tolkien never finally decided whether King Gil-Galad was a descendant of Fingolfin or Finarfin; in the published Silmarillion he's the former, but in Tolkien's latest drafts he's the latter.
- In Supernatural, Sam and Dean have spent most of their twenty-some years with only each other as dependable family. Eventually, in the episode entitled "Jumping the Shark" including references to 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 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 as himself, after he's been brought back from the dead as bait or a 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 save Sam's soul from eternal torment in Lucifer's cage or rescue Adam's from the same place, Dean unsurprisingly doesn't even hesitate in choosing Sam.
- Simultaneously, within the archangels, Lucifer and Michael are seeking the same Armageddon, each thinking they would win. Winchester brother intervention gets Gabriel involved, introducing this dynamic between them to the viewers. Gabriel takes neither side, choosing humans instead, and is responsible for telling Sam and Dean how to seal the both of them in Lucifer's Cage, preventing the conflict that would raze the world.
- Charmed revolved around three sisters, the 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 Ninja Turtles: The Next Mutation series was a female turtle with the same Meta Origin. But she got so much hate from the fans and creators of the original series, and the show got terrible ratings and was cancelled, so the whole thing is 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. 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 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 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, The Reveal 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.
- Peaky Blinders: The introduction of Michael Gray—Polly's son, the boys' cousin—complicates the already-fragile relationships among the Shelbys. In particular, John and Polly square off on whether or not allowing Michael to participate in the less-legal side of the business is a wise idea.
- F.E.A.R.3's Third Child is hinted in the good ending to have a brighter future than its brothers.
- In Mortal Kombat: Armageddon Rain's Ending it is revealed he is the third son of Argus - the other two being rivals Dagon and Taven
- Team Fortress 2's Gray Mann not only twists the family dynamic like Seth, he one-ups Cain, and murders both his brothers.
- Fire Emblem: Genealogy of the Holy War ends up with a show down between 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, 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 Metal Gear Solid, 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 the FOXDIE disease carried by Snake. However, as The Stinger in Metal Gear Solid alluded to, there was a third "Snake brother", Solidus Snake. He shows up in Metal Gear Solid 2: Sons of Liberty 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 Well-Intentioned Extremists.
- Blazblue have Ragna, Jin and their sister Saya. Saya was kidnapped 10 years before the start of the story and is nowhere to be seen in the present, both Ragna and Jin assuming that she was long dead. The the ending of the second game came and revealed that she was very much alive and now the head of The Empire subjugating the world, additionally she's also possessed by a death god. So both brothers have to put aside their own animosity for each other and deal with their sister.
- In The Venture Bros., Dermott Fictel is introduced, and almost instantly separates Dean and Hank, as he is more similar to Hank, following the brothers' Divergent Character Evolution. 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.
- In The Miser Brothers' Christmas, a 2008 sequel to The Year Without a Santa Claus, the squabbling title characters have to call a truce to confront their even nastier older brother, the North Wind.
- Downplayed or averted in Gravity Falls—the relationship between the "original Mystery Twins," Stan and Ford, turns out to be a major part of the series, while their third sibling, Shermie, is only mentioned in passing. Justified because he was a baby when the other two left home; from a Doylist perspective, he only really exists to explain where Dipper and Mabel came from.
- On Daria, the title character's mom, Helen, has an intense rivalry with her older sister, Rita, due to her perception that Rita has always been their mom's favorite (though Rita actually seems to think that Helen is). The youngest sister, Amy, describes how she always felt like she was ignored and thus grew more independent...but as we see, she can be just as petty as the others, and deftly works to play them against each other.