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[[quoteright:350:[[WesternAnimation/MartinMystery http://static.tvtropes.org/pmwiki/pub/images/rsz_135_full_6815.jpg]]]]
[-[[caption-width-right:350:Martin's feeling the step-brotherly love...in his pants.]]-]

->'''Greg Brady:''' No, we wouldn't go to hell, because she's not really our sister.\\
'''Peter Brady:''' So these feelings are natural?
-->-- ''WesternAnimation/RobotChicken'', "[[Film/MrAndMrsSmith Mr. and Mrs.]] [[Series/TheBradyBunch Brady]]"

So you've got two healthy, unrelated teenagers whose parents have recently gotten hitched. Said teenagers happen to be of the opposite sex. Whether they find it {{squick}}y or not, hormones are going to fly.
Usually this is seen in the undertones of their behavior in a show, and it almost always happens when two people become step-siblings in adolescence. Sometimes though, this is unintentional on the part of the writers and attempts to make new step-siblings act like a long-time brother and sister [[RelationshipWritingFumble just come off as flirty]].

The RomanceNovel is also fond of this trope, generally involving older stepsiblings. It has the double effect of giving them reason to resent each other over their lives' disruption and giving them no way to escape the other entirely (without cutting off from their [[ThickerThanWater parent]] as well).

Sort of a vanilla version of NotBloodSiblings. See LittleSisterHeroine for when the relationship is made more explicit and emphasized.

People who become step-siblings at a very young age and grow up together are more likely to consider each other truly siblings and thus off limits for dating. (Plus, if it happened at a young enough age, the WestermarckEffect may be involved.) Since the instinctive biological aversion to mating with a sibling or childhood friend is highly dependent on actually being raised in close proximity to said sibling/childhood friend, this can be TruthInTelevision for many 'siblings' whose parents married during or after their puberty, though they rarely end up becoming an official couple due to social factors.

----
!!Examples:

[[foldercontrol]]

[[folder: Anime and Manga]]
* The manga ''Yoru Made Matenai'' is built around this trope, with a wee bit of borderline ShotaCon fanservice.
* The main couple of ''MarmaladeBoy'', which starts with the two leads's parents switching partners and then all deciding to live together in one BigFancyHouse.
* The whole point of ''AkumaDeSourou''. Kayano and Takeru are in high school when their parents get engaged and start living together. Takeru heavily pursues Kayano, and naturally she gives in. Things get... complicated... as Kayano tries to keep their relationship secret to avoid breaking up their parents'.
* Though not teen-aged, Erio and Caro from ''Anime/MagicalGirlLyricalNanohaStrikerS'' were both technically adopted by Fate and, as shown in the companion manga, think of themselves as step-siblings. However, their relationship is so close as to be romantic that even other people in-universe [[ShipperOnDeck tried to push them toward each other]]. Pretty much becomes official as of ''Force''.
* This is the starting point of an h-manga titled ''Giri Giri Sisters'': [[{{Tsundere}} Mion]] has fallen in love with her classmate Tsukasa, but [[CanNotSpitItOut before she gets up the nerve to confess]], her widowed or divorced mother marries his widowed or divorced father. Compounding the problem, her younger but much curvier sister Kotone is also hot for their new stepbrother, and not at all shy about saying so and demonstrating it. The good news is, Kotone's not just willing but eager to [[{{Polyamory}} share Tsukasa with her big sister]]....
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* ''kiss X sis'' is about two twin sisters who love their stepbrother very much. Their mom and the stepbrother's dad approve. He doesn't. HilarityEnsues.
* Another rated manga example, ''Ai Don't Koi'' from Yasuhira Nakanishi. The main character struggles between two choices: one is his UnluckyChildhoodFriend whom he always thinks of as a little sister, another is the girl whom he had a gigantic crush on - until her mother married his [[TheCasanova father]].
* Yuuka and Atsuro Kiryuu in ''Manga/SensualPhrase'', though only in the manga. One of the side-stories [[ADayInTheLimelight focuses on this]], [[spoiler: and specially when Riri Kinoshita, an IdolSinger who has a crush on Atsuro, blackmails him into pretending to be her boyfriend under threats of spilling out the beans about their "incestuous" bond, which would ruin Atsuro's budding career. [[WhatAnIdiot And then she pisses him off via verbally abusing poor Yuuka]]...]]
* ''Manga/CherryJuice'': As the series progresses, step-siblings Minami and Otome often switch between sharing flirtatious romantic moments and BelligerentSexualTension.
* Subverted in a case of ''Manga/DetectiveConan''. [[spoiler: A woman named Kimie had to give her toddler daughter Yukiko away to her husband's family due to [[BreakTheCutie the terrible circumstances she was going through after her husband was in jail]], and didn't know what had happened to her for many years. In the meantime, she got married to a man named Tsujimura, who had just gotten divorced and had a son named Takayoshi. Several years later, Takayoshi falls in love with a girl and brings her photo to his dad and stepmom... and the girl turns out to be Yukiko, who doesn't know anything about it. Tsujimura has a FreakOut and [[ParentalMarriageVeto forbids Takayoshi from dating Yukiko]], having recognized the girl's identity as his wife's estranged child... which then leads [[MurderTheHypotenuse to a]] [[TheUriahGambit horrifying reveal]].]]
* [[spoiler: Kazuma and Tsubasa]] from ''KareKano''.
* Eren and Mikasa in ''Manga/AttackOnTitan'', to the point that ''other characters'' mention it.
[[/folder]]

[[folder: Film]]
* ''Film/CruelIntentions'' ramps this trope UpToEleven, with the plot driven by a girl promising her besotted stepbrother sex if he does what she wants. ''Film/NotAnotherTeenMovie'' parodies this with them actually being related, and the brother being utterly repulsed at the idea, but going along with TheBet for other reasons.
* ''Film/{{Clueless}}'', of the BelligerentSexualTension variety.
* ''AVeryBradySequel'' approaches this trope when Greg and Marcia are reminded that they actually aren't at all related.
* ''Drive Me Crazy'' has a variant of this -- [[spoiler: the two teenagers have a mostly-normal romance, and their parents get married at the ''end'' of the movie.]]
* The horror movie ''Film/{{Teeth}}''. The brother has spent most of his life with a crush on his step-sister and despises their parents for making them siblings.
* The comedy ''Film/LoneStarStateOfMind'' (produced by Edward Zwick, of ''Film/{{Glory}}'' and ''MySoCalledLife'' fame) has stepsiblings who are engaged. They had been dating when their single parents married each other and continued the relationship after that.
* This is brought up ''On a Clear Day You Can See Forever''. Todd Pringle (played by JackNicholson) points out that, since he is just as much in love with Daisy (played by BarbaraStreisand) as Doctor Chabot is, perhaps he (Todd) might make a suitable match for her. When Dr. Chabot points out that Todd is Daisy's brother, Todd responds with, "Only by marriage... she's my step-sister, not my blood-sister."
* Kind of in ''Literature/ThePerksOfBeingAWallflower''. Sam and Patrick are initially mistaken for a couple by Charlie as they are so comfortable around each other, but they laugh when he asks how long they've been together and reveal that they step siblings. Not only that, but Patrick is gay as well.
* In ''LoversOfTheArcticCircle'', Otto's divorced father marries Ana's widowed mother first when the two children are just ten years old. Then, as they are living together in the same house, a teen romance gradually develops between Ana and Otto into a full-blown sexual relationship which, however, they manage to keep completely secret from their parents/step-parents to the end.
* Nemo and Anna in ''Film/MrNobody'' started off as ChildhoodFriends who later become step-siblings and start developing a romantic relationship.
[[/folder]]

[[folder: Literature]]
* Blair and Aaron in the ''GossipGirl'' books.
* In ''Literature/{{Twilight}}'' all the Cullens are adopted siblings and most of them are in a relationship with another Cullen, though the siblings thing is largely an act to explain why they move and cohabit together.
* Julia and Tiberius in ''Jeg elsket Tiberius'' (''I Loved Tiberius'').
* Harry and Elaine from ''Literature/TheDresdenFiles''. They were both adopted by their EvilMentor in their early adolescence, having no known genetic connection. Once puberty struck they took their relationship in the expected direction. [[spoiler: for extra squick and horror, their mentor secretly encouraged it to keep them isolated from their peers, paving the way to magically brainwashing them into being his enforcers.]]
* ''Big Trouble'' by Dave Barry subverts this. Matt and Jenny have a strong crush throughout most of the book. When their respective dad and mom hook up in the epilogue, however, they only go out on a few dates before realizing the whole thing's a little weird and deciding to just be friends.
* In the ''AmeliaPeabody'' books, Ramses and Nefret were raised as siblings from the ages of ten and twelve, respectively, but ended up falling in love and marrying, albeit with some complications.
* ''{{Literature/Malevil}}'' has Jacquet and Miette. They are lovers as well, but Jacquet has to [[EthicalSlut share her]] with [[{{Polyamory}} every other man]] at Malevil.
* A technical version occurs in ''Literature/SongOfTheLioness'', when Alanna's [[ParentalSubstitute adoptive father Myles]] falls for George Cooper's mother Eleni and they marry. Technically this means that Alanna and George are step-siblings when ''they'' rekindle their romance and get married, but given that they're adults and the other circumstances, the trope isn't played at all.
* Even though the boy comes into the family through adoption rather than marriage, ''Literature/WutheringHeights'' fits the rest of the trope, and is what happens when this sort of relationship goes spectacularly badly wrong. (Although there is a school of thought that believes that Catherine and Heathcliff are actually half-siblings.)
* The whole point of ''Literature/BrothersConflict'', specially its visual novel counterpart.
[[/folder]]

[[folder: Live Action TV]]
* ''LifeWithDerek'': This seems to be unintentional on the writers' part, but very intentional on the actors'.
* ''Series/{{Lost}}'': Boone is in love with his step-sister Shannon. Shannon is aware of it and frequently uses it to take advantage of him, [[spoiler: culminating in them having sex followed by her blowing him off once she's gotten what she wanted]].
* ''TheSecretLifeOfTheAmericanTeenager''
* ''{{Reaper}}'' has Sock getting a stepsister. She thinks of them LikeBrotherAndSister, but tends toward much IDidntMeanToTurnYouOn. ''He's'' trying to get in her pants. ([[{{squick}} He eventually succeeds]].)
* ''Los Serrano'' has the relationship in between Marcos and Eva. {{Lampshaded}} in a dream sequence ([[spoiler: [[ItWasHisSled despite the series itself being]] AllJustADream, but anyway]]) where Marcos says their relationship isn't incest, "more like stepincest".
* From the Taiwanese drama ''DevilBesideYou'', Qi Yue was being bullied by the son of the school's president, Jiang Meng, only to [[AllGirlsWantBadBoys fall in love with him]], after realizing that he was a JerkWithAHeartOfGold and actually a decent guy. Before either of them could understand their feelings, her widowed mother and his divorced father began dating, eventually becoming engaged. One of the main conflicts of the story is the main couple trying to work out their relationship amidst their parents' relationship as well.
* Happened in the regular ''TheBradyBunch'' too. Greg and Marsha have to share a room in the attic with only a bedsheet seperating the room. Greg ([[{{Fanservice}} and the audience]]) watches her silhouette change behind the sheet, and the {{UST}} is on.
* Morgana and Arthur in ''{{Series/Merlin}}'' had a [[{{UST}} very charged]] relationship in Season 1.
** "The Crystal Cave" [[spoiler:reveals that Arthur's father Uther is Morgana's biological father, having had an affair with Gorlois' wife Vivienne]].
* ''Series/{{Glee}}'': Now that their parents are married, Kurt and Finn. It certainly didn't help that the title of the episode was "[[FanNickname Furt]]". Finn even sang a song for him! Of course, Kurt originally tried to get Burt and Carole together in an attempt to [[InvokedTrope invoke this]], as he had a crush on Finn for months beforehand ([[IncompatibleOrientation it didn't go very well]]).
** In every episode thereafter, however, well and truly averted; Kurt's interactions with Finn are nothing at all like his flirty, adoring interactions with his boyfriend Blaine, and the two stepbrothers seem to be uninterested in each other much of the time, and even vaguely dismissive. It's unsurprising given that Kurt got a double dose of [[JerkJock Finn's real personality]] in 1x20 and 2x04, and that would be enough to kill ''anyone's'' interest. Except Rachel's, of course.
* ''TheFreshPrinceOfBelAir'': Inverted. Will's mother and Lisa's father take a liking to each other, and after their kids decide (at the altar) that they're not ready to be married, their parents step up, saying "[[{{WhyWasteAWedding}} Why waste a perfectly good ceremony?]]"
* The German comedy series ''Türkisch für Anfänger'' ("Turkish for Beginners", 2006-2008) is about the family consisting of therapist Doris Schneider, her children Lena and Nils, her lover, later husband, police inspector Metin Öztürk, and his children Cem and Yagmur. One of the major plotlines of the series is the on-again-off-again romance between stepsiblings Lena and Cem. [[spoiler: At the end of the third season the two are a couple and Lena is pregnant.]]
* On ''Series/{{Degrassi}}'', Clare dates her mother's fiancé's son Jake. Later on, when they are thinking about having sex, Jake says they should just end the relationship because if they break up painfully it would get awkward, seeing as they are now related.
* ''Series/ModernFamily'' has a Flirty Step-Uncle. [[KidAnova Manny]] develops a one-sided crush on his (older) step-niece, Haley. He also makes a few flirty comments to her sister, Alex. Both of the sisters are thoroughly disgusted by this.
** This has dropped in more recent seasons, but now Manny's adopted younger step-niece, Lily, has a crush on ''him''.
* In ''Series/GossipGirl'', Serena and Dan find themselves in this situation when their parents, Lily and Rufus, marry each other. [[spoiler: After they divorce, Dan and Serena get together, and end up marrying in the finale.]]
[[/folder]]

[[folder: Video Games]]
* ''VideoGame/ValkyriaChronicles'': The main character makes his "Adopted" sister's non-blood relation clear to a very suspect degree.
* [[EvilMentor Master Li]] in ''JadeEmpire'' runs a martial arts school, and his two prize students ([[TheChick Dawn Star]] and the [[PlayerCharacter Spirit Monk]]) both are his adopted children. [[spoiler: After certain revelations about Master Li, it becomes apparent that there is also a case of [[KissingCousins flirty stepcousins]] if the Spirit Monk enters into a relationship with Silk Fox.]]
[[/folder]]

[[folder: Webcomics]]
* The very last strip of ''Webcomic/OzyAndMillie'', with a few hints shortly prior, indicates this is going to be how things are for the title characters.
[[/folder]]

[[folder: WebOriginal]]
* Mickey and Audrey on ''{{HomeSchooled}}''. The two usually spend their time bickering, so this could be BelligerentSexualTension / SlapSlapKiss.
[[/folder]]

[[folder: Western Animation]]
* ''WesternAnimation/{{Sixteen}}'' has Jonesy and Jen in spades; they even kiss and Jen admits she used to have a crush on him.
* ''WesternAnimation/MartinMystery'': It takes the subtext to the nth degree. [[GettingCrapPastTheRadar They were really pushing it to the limit.]] That's what happens when you take characters who were lovers in the original comic and [[SiblingSwitchSquick turn them]] [[DubText into]] NotBloodSiblings.
* In one episode of ''WesternAnimation/TheSimpsons'', Homer is opposed to the thought of his dad marrying Marge's mom,
-->'''Marge:''' Homer, what possible reason could you have against Grandpa being in love?
-->'''Homer:''' If he marries your mother, Marge, we'll be brother and sister! And then our kids - they'll be horrible freaks with pink skin, no overbite, and ''FIVE FINGERS ON EACH HAND!!''
[[/folder]]

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