A common cop-out in anime, manga and soap operas, when a series is based on Brother-Sister Incest, is to reveal towards the end, or explicitly state in the opening monologue, that they're Not Blood Siblings. Usually, for all intents and purposes, everyone acts like they are. Alternatively, it can be used to explain why everyone's okay with the relationship.
This can also be foreshadowing for such a relationship, as many shows usually won't make a big deal about adopted children unless there's a Changeling Fantasy involved.
Sometimes this is a result of Bowdlerization, Media Watchdogs, and Executive Meddling, in that in the original series they were blood relations. It also comes up in Moe, for fans who, while probably not having siblings, still feel a bit too squicky about it. First Girl Wins can sometimes explain the prevalence of this trope.
This trope is often related (but not by blood, of course) to Flirty Stepsiblings and Little Sister Heroine. Needless to say, these tropes tend to gross out anyone who actually has stepsiblings.
In older works, when sex shouldn't even be hinted in family friendly stories, main child characters are usually conveniently adopted orphans. That way, the main male and female character don't have to be accounted as their actual parents. This situation does lead to a lot of confusion with readers. A boy and a girl are adopted by a family, therefore appear to be brother and sister, but still have a romance thing going on and jealousy when one of them has success with another love interest. Since the situation isn't always explained, this often leads to disturbing, confusing ideas with the readers.
Quite often it's just a case of cultural misunderstanding. In Japanese the words for "big brother" (兄さん - nii-san) and "big sister" (姉さん - nee-san) are very commonly used in casual speech to address young people in general, too, and it's not rare that little kids and teenagers will use them to address to older but still young friends (or love interests). However, sometimes clarification is needed if the context is not clear enough about it.
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Pristine example in Miyuki: The main character soon discovers that his stepsister Miyuki is not blood-related to him. She ignores it until the end of the manga when he proposes in the middle of her wedding to one of his best friends. She then breaks up the wedding, and they eventually get married.
Marmalade Boy, although Yuu does find some circumstantial evidence suggesting that they are blood siblings, and quickly breaks up with Miki, gets the hell out of town, and leaves them both to suffer. They finally conclude that they still love each other and want to get married, but their parents reveal that no, it was just a misunderstanding.
The original proposed ending would have found them as siblings after all. They would have broken up in that ending, and Miki/Ginta and Yuu/Meiko would have formed.
Love Hina introduced the hitherto unmentioned Kanako, Keitaro's quasi-adopted sister. Ironically Keitaro is the only character who never rationalizes Kanako as being 'fair game', while Naru ends up taking his role of clumsy awkward relationship handler for a period of time because she does.
Tetsuya and Jun from Great Mazinger were two orphans taken in by Kenzo Kabuto and were raised together. Technically they are adoptive siblings, but they are not related by blood at all, so no one bats one eyelash regarding their ongoing Belligerent Sexual Tension.
In Hot Gimmick, Hatsumi is unaware for a large portion of the series that her brother Shinogu is adopted. Shinogu, however, is aware and even moves out of their family's apartment as part of a plan to make Hatsumi see him as more than a brother. His compassionate nature also arguably makes him a preferable match for Hatsumi, compared to her Jerkass boyfriend Ryoki.
The novel has a Shinogu/Hatsumi ending, for those of us who thought the manga's ending was TEH FAIL.
Yoake Mae Yori Ruriiro Na has two sisters for the lead, neither of whom is his blood sister, and both of whom are haremettes in the game. (For the record, Sayaka is his cousin, and Mai was adopted).
... instead choosing to briefly tease a potential relationship between Fate and her brother (by virtue of his mother adopting her) Chrono. Although this too was abandoned in the third season when Chrono married his childhood friend, Amy.
Speaking of the third season, in Magical Girl Lyrical Nanoha Striker S the hinted-atToy Ship of Erio and Caro may sort of fit this in that Fate ends up being the legal guardian of both. The accompanying StrikerS manga has them repeatedly state that they are technically adopted siblings... while lathering on even more Ship Tease. Unlike other non-blood siblings, they had never met prior to StrikerS, and had no pre-existing sibling relationships (they even begin on a Last Name Basis).
Hiro and Karada in Asatte No Houkou; not taken to a romantic end, but still important to the plot.
Has the heck spoofed out of it in Ninin Ga Shinobuden (a.k.a. Ninja Nonsense), where one of the main characters, a yellow Pac-Man-esque anthropomorphic football who is really a Ninja Elder refers to this particular trope and, being a pervert, has a particular high esteem. "If the Master says there is a cute little non-blood related sister, there IS a non-blood related sister."
Turns out Touga is her brother. They were both adopted.
Pointed out in the visual novel-based OVA _summer, where Osamu, the typical idiot best friend character, points out "Sana's technically your stepsister, so you could go for her if you want..." when looking over the many girls the protagonist has gathered around him.
Well, when one considers that she was only "adopted" a couple years ago (ie after Kai had hit puberty) and looked like a teen girl already, it's not so strange.
Code Geass played with this in one of its many Sound Episodes. Shirley goes through a series of Imagine Spots in which her attempts to ask Lelouch out go spectacularly wrong. In one of the imaginary sequences, his Ill Girl little sister Nunnally enters the scene and says that they'll be late for the wedding; as Shirley recoils in shock, the pair claim that they're Not Blood Siblings, so it's okay.
In the manga Ani-Com by Yuu Yabuuchi (writer of Naisho No Tsubomi and other shojo manga), this is the basis of the plot. It's played out that the remarrying parents expected their two children to get together and get married, and they do.
Completely inverted in Koi Kaze, where Koshiro and Nanoka develop an attraction for each other and then find out they are blood siblings who haven't seen each other in 10 years. The story revolves around how they cope — or not — with the fact that they can't get over their feelings for each other.
In fact, Koi Kaze uses the documented phenomenon of "genetic sexual attraction" as a factor in Koshiro and Nanoka's interactions.
Interesting variation in the manga Boku No Futatsu No Tsubasa when Hiromi gratuitously reminds Makoto (after a lot of borderline incestuous behavior) that they aren't blood cousins, it foreshadows the reveal that Hiromi's adoptive mother is Makoto's biological mother, meaning they aren't blood sisters, either.
In the Okazaki Tsuguo manga Justy, the titular character has two of this sort of sisters (the older one "adopted" him when she found him as a lone child, and then he "adopts" the younger after he kills her criminal ESPer father), and they both have crushes on him. Justy ends up romantically involved with the older one, Jerna. And he continues calling her "neesan," even when they're nude and postcoital in bed together.
In one of the few non-romantic angles, in One Piece it is revealed that Ace and Luffy aren't biological brothers. Ace is actually the son of Gold Roger. Not that such things stop them from treating each other as true siblings, such as facing incredible risks to protect one another.
The two have another "brother," Sabo, who was killed by the Celestial Dragons while trying to go out to sea. According to Ace, sharing a drink makes people brothers.
Like the above, Toshiro Hitsugaya and Momo Hinamori from Bleach aren't a romantic couple (although a good chunk of the fandom believes otherwise), but they were raised together and treat each other as actual siblings. Hitsugaya, however, is just a liiiiiittle bit overprotective of Hinamori, but that's an entirely different trope you're dealing with.
None of the Elmore kids in Psyren are blood-related. Probably why Fredrica and Marie is one of the most common pairings.
Cherry Juice features a boy and girl of the same age becoming step siblings as young children, who eventually grow to have the hots for each other as teenagers. Their relationship is supported by both the girl's friends and her grandmother, who does everything she can to get the two together.
The manga, Me And My Brothers, takes great pains to remind the reader nearly every chapter that the heroine is in fact not blood-related to the titular brothers, despite the also-incessant reminders that they are all family and that Sakura is their "little sister." This is used quite blatantly so Sakura can be obsessively fawned over by her new guardians—and eventually fall in love with one of them—without the squick.
In ROD the TV Michelle, Maggie, and Anita are "sisters in law" rather than actual sisters, though this is more for plot reasons than pairing them together.
The main plot of Akuma De Sourou; Takeru and Kayano's parents are engaged when they begin their relationship, so understandably they Kayano goes to great lengths to keep it secret. Some people in universe accept it, and others don't.
Never stated outright in Axis Powers Hetalia, but the amount of Ship Tease between characters that aren't technically related by blood is astounding, as adoptive older brother figures tend to later become possible Love Interests (in fans' eyes, at least). Possible couples include: Switzerland and Liechtenstein, China and any of the Asians, any arrangement of England/France and America/Canada, possibly Spain and Romano... And because this universe is rather unclear about how exactly countries are "born," fans can and do decide whether or not certain couples are incest based on their preferences.
Averted with Norway and Iceland - the latter's "DNA testing results" (archaeological excavation) prove that they are, in fact, related.
Manga scenarist Shirodaira Kyou apparently loves playing with this.
A variation in The Record Of A Fallen Vampire: Akabara's adopted daughter fell in love with him, though he still sees her as his little girl. As a bonus, Akabara later married her half-sister.
In Spiral, Ayumu is strongly hinted in the anime and confirmed in the manga to be in love with his sister-in-law. Inverted with Kousuke and Ryouko, who have blatant Ship Tease much before we learn (in the manga only) that they're actually half-siblings — but they've always known it, and though they've been raised apart, it has always been an obstacle to their relationship. (They eventually do make it to Official Couple status.)
In episode 19, it is been revealed that Himari is not Kanba and Shouma's blood sister; she is actually a girl rescued from the child broiler and was adopted into Kanba and Shouma's family, And she is in love with Shouma.
In the next episode, this is also revealed Kanba is not really Shouma's blood brother as well.
Vampire Game has Laphiji and Seilez, although, as Ishtar has said, the family's so inbred everyone's practically married to their siblings anyways.
At one point in Fire Tripper, Suzuko falsely believes that she and Shukumaru are brother and sister by blood. When she discovers that the two of them actually aren't blood-related, she happily returns his affections and marries him.
In Family Compo Masahiko has feelings for Shion, who acts as his sister and is referred to as so. But while Shion isn't his sister, she is his cousin.
In the OVA adaptation of the bishoujo game Ko-ko-ro..., Souji is shown in flashback to be adopted by the Kuonji family. This is apparently not lost on his younger sister, Asuka; though she addresses Souji as “Aniki” (Japanese for “older brother”), she regularly initiates intercourse with him and secretly watches him having sex with other girls in his bedroom.
When in public, Asuka is openly affectionate and quite clingy with Souji, acting much more like his girlfriend than his sister, and seems to relish the shocked stares of people who see them together.
The OVA scenario is about as shocking as in the revised version of the game, in which Souji and Asuka were merely stepsiblings, but neither is as shocking as that of the original release of the game, where they actually were blood siblings.
Jeremy and Ian of A Cruel God Reigns fall into this category. While not blood brothers, they are brothers through adoption.
In ElfQuest, it is hinted at in the comic, and expressly stated by the Word Of God, that Cutter and Skywise have had sexual relationships. It is also mentioned in the comic that they are "brothers in all but blood." Of course, the elves are said to not have the same concept of relationship as (modern Western) humans do, even though nearly all of them are paired in "life bonds" (i.e. marriages).
Mister Immortal from the Great Lakes Avengers learned he was immortal after his stepsister's death, which became Narmtastic after his repeated botched suicides.
In X-Men comics, Nightcrawler's "true love" is his adoptive sister Amanda Sefton, even though they were raised together since infancy. They actually did officially get together, late in Excalibur's original run (and was portrayed as a bad idea - not because of their prior relationship, but because Amanda was portrayed as not-quite-right in the head). Nightcrawler (as Mystique's blood son) and Rogue (as her adopted daughter) have potential for this, as well. Which if it happened would be less potentially squicky than Amanda; they never even met until adulthood. He still flirts shamelessly with her.
Back in early 1980s, DCU stories set on Earth-Two hinted at mutual attraction between the Huntress (Helena Wayne, Earth-Two Batman's daughter) and the local version of Dick Grayson. It pretty much remained subtext, with both parties insisting all too earnestly that even though they weren't biologically related, their relationship would be too wrong to even contemplate. That subtext inexplicably became text in the recent Justice Society of America Annual.
One problem: The 1980s version was from the original Earth Two. After Crisis on Infinite Earths, that Universe ended. The whole point of the recent Annual was to show that the Earth 2 Power Girl was sucked into was NOT the same Earth 2 she came from.
It is heavily implied that Despair II has an unrequited crush on her adoptive brother Destruction in volume 7 of The Sandman.
Actually brought up (in a non-sexual way) during The Avengers: Thor is angry when the other Avengers start talking about how evil Loki is, claiming that "he's still of Asgard, and my brother." Upon hearing just how many people he'd killed in the past few days, he seems to take this back, saying "He's adopted."
Happens not once but twice in Gerard Way's comic series The Umbrella Academy; first between the Rumor and Spaceboy, and then the Kracken's obvious unrequited feelings for Vanya, as well.
In an early Alpha Flight story, it's strongly implied that Marrina's adopted brother was in love with her.
Suske En Wiske: Suske and Wiske are both adopted by their aunt, Tante Sidonia, and thus raised as brother and sister. Still, many readers have had the impression that they actually WERE siblings. This caused a lot of disturbing scenes where Wiske is jealous of Suske's success with other girls and a case of Brother-Sister Incest appeared to be going on.
Nero: Petoetje and Petatje are also adopted and raised as brother and sister by Madam Pheip.
In Prince of Persia: The Graphic Novel, Layth and Guilan are married before the story begins. They were raised as brother and sister after Saman, Guilan's father, defeated the Caliph's army and killed Layth's real father in battle.
Phineas and Ferb often fall into this; they're canonically stepbrothers, but refer to each other as brothers and have been raised together since they were toddlers. Slightly less commonly they'll be paired with their older sister (biological for Phineas, step- for Ferb) Candace.
A fairly popular fan theory is that Doofenshmirtz is Phineas' biological father. Given that Ferb canonically has a crush on Doofenshmirtz's daughter Vanessa, some have noted the strangeness that Phineas' (step-)brother would be dating his (half-)sister, even though Ferb and Vanessa wouldn't be siblings by either blood or adoption.
Dib and Gaz from Invader Zim are brother and sister, but Word Of God hints that Dib may have been an experiment created by their scientist father (and, many fans guess, Gaz too). Some have argued this makes them a viable shipping pair. It's not a very popular ship, though.
Derek and Casey of Life With Derek is a non-canon ship that's largely popular with the fanbase, as opposed to all the other love interests that are introduced throughout the series. The fact that half of the show focuses on the interactions and the relationship between the two step-siblings doesn't help.
As a sort of BetaFan Preferred Couple of sorts, there's also Edwin and Lizzie, the younger siblings of Derek and Casey respectively. Doesn't help that both sometimes use the closet as a meeting place, which immediately has shippers thinking of the Ten Minutes in the Closet trope.
In the Gender Flip-based John Lane fanfic series, this happens in a more justified situation — After Helen learns how totally neglectful John's parents (Trope Namers of Casa Lane Parenting) are, she takes them to court and takes custody of John Lane, who at that point is already in a budding relationship with Daria.
Mari tries to use this argument with Uri in Nobody Dies; he shoots her down regardless, still squicked by the prospect.
In one Naruto fan comic, Temari discovers her father's secret papers, which prove that she was actually adopted, and doesn't have to hide her feelings for her younger brother Gaara any longer. Kankuro then wonders if he can do the same for Gaara, but Temari beats him up while reminding him that he and Gaara are still related, since she's the one who was adopted.
Oneiroi Series: Terentius briefly tries to use this rationale to excuse himself from wantinghis adopted half-sister, but he quickly breaks down, admitting to himself that it really doesn't matter and that he was lusting after his little sister.
Subverted in Relationships Series. When it comes to light that Nove and Ginga, who are in a three-way relationship with each other and Wendi, are genetically related, this revelation motivates Genya's decision to send Ginga on a mission to separate her from Nove and Wendi.
In Legolas By Laura, the eponymous elf adopts the heroine Laura as either his sister or his daughter (it's not really made clear). After rescuing her from orcs, he agrees to "be your boyfriend," even though she's both his adopted sister and ten years old.
The Star Wars fanfic "Father and Son" has Han and Leia as a couple (as in canon)... despite the fact that her (and Luke's) birth parents adopted and raised Han for several years before the twins were born. Somehow, nobody (not even Vader or Padme's extended family on Naboo) sees anything wrong with the two being a couple. (Admittedly, all three siblings were sent to separate planets after Anakin's Face Heel Turn and Padme's Death by Childbirth, and didn't find out about their being siblings until after they were all adults. But still.)
Thor and Loki of the Marvel universe tend to be the subject of some arguments regarding whether or not pairing them constitutes incest. On the one hand, they were raised together and usually refer to each other as brothers (albeit somewhat sarcastically), but being that Loki is an adopted frost giant and goes by Laufeyson and not Odinson, whether or not they both see each other as siblings depends on the writer.
The Spanish movie Los Amantes Del Círculo Polar extends this trope from Flirty Step Siblings in that 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. Curiously, the possibility that the relationship between two opposite-sex step-siblings could ever go beyond familial love never does seem to occur to either of their parents/step-parents or much of anyone else, which is why the two are able to conceal their nightly liaisons from them so easily.
A very clumsy Western example: Ator The Fighting Eagle (the first Ator film; MST3K fans might be more familiar with its sequel Cave Dwellers). Ator, the titular swordsman, falls in love with what he thinks is his blood sister and when he asks their father permission to marry her, the father happily reveals that he is in fact, adopted. (Instead of, you know, being totally squicked out by the idea that he would love his blood sister in the first place).
Richie and Margot in The Royal Tenenbaums are a somewhat more realistic treatment, as their relationship is considered inappropriate despite everyone knowing that Margot is adopted.
We mean everybody. Royal introduces her to everybody he knows as "my adopted daughter, Margot Tenenbaum."
Clueless, where Cher realized at the end of the movie that she had been unconsciously harboring feelings for her former stepbrother. (Their parents were already divorced by this point).
In Cruel Intentions the main character Sebastian is obsessed with his step-sister Kathryn, who uses the promise of sex to get him to help her with her revenge.
This was brutally parodied in Not Another Teen Movie when their Kathryn expy constantly tries to seduce her brother.
Jake: That's disgusting. You're my sister!
Catherine: Only by blood!
The Wrong Box: Parodied in the Dudley Moore film: when the protagonist laments that cousins cannot marry, his love interest replies that she was adopted. After a moment's reflection, he responds: "Really? I was adopted too."
The Lion King has constant debates on how, if Scar and Mufasa are the lions of the pride, Nala is whose daughter exactly? Making her what relation to Simba?
There's also debate on whether Shenzi, Banzai and Ed are related, partly for the purpose of writing fanfiction with the Shenzi/Banzai pairing and partly because it's one of those things it's fun to argue about.
In The Lion King 2, isn't it so convenient that Kovu isn't really Scar's son, making it perfectly okay for him to get it on with his cousin?
The French movie The Flower Of Evil has a pair of stepsiblings who have had the hots for each other for years. Nobody really thinks this is squicky, in fact some of the family seem to wish they'd just get on with it.
In Big Trouble, Matt Arnold and Jenny Herk's parents meet through each other, have sex, and eventually get married. Meanwhile, their relationship is somewhat implied to be a bit more than platonic.
In Teeth, Brad likes his stepsister a little too much. She doesn't know it for most of the time and is horrified when she finds out.
In Lone Star State of Mind, Junior and Baby were already dating when his mother and her father decided to get married. Since they're not actually siblings, they don't see anything wrong with their relationship. A few people still harass them about their technical incest.
In the Hammer Horror picture Paranoiac, a rather mentally-unstable woman falls in love with a conman impersonating her long-dead brother in order to get some inheritance. She is rightly horrified by this, but feels better when it turns out he's not really her brother, and eventually forgives him for the con. They're not portrayed as particularly likable characters, of course, so our sympathies end up with the Villain Protagonist other brother, Oliver Reed, who is trying to bump the pair of them off (having already killed the guy being impersonated) so he can have the whole inheritance to himself.
In "Princess Belle-Etoile", Princess Belle-Etoile is found with her two brothers and her cousin as a baby and they are all raised by the same couple. Princess Belle-Etoile and her cousin fall in love and find it very strange that she doesn't react to him the way she does to her brothers. Then, they learn they are foundlings and set out, and so find that they are only first cousins, and marry.
Antigone and Haemon in Antigone. Of course, Antigone's mother is also her grandmother, so being into Haemon seems almost normal by comparison.
At least kind of; Elizabeth was raised by Victor's parents, but there seems to have always been a kind of barrier between the two thinking of each other as siblings, probably because their family always wanted them to get together as adults. Note that they may be biological cousins depending on which edition you read.
Heathcliff and Catherine Earnshaw in Wuthering Heights (1847), maybe. The book leaves it ambiguous as to whether or not Heathcliff is actually Mister Earnshaw's bastard son, and thus, Catherine's biological half-brother.
Subverted in Dave Barry's novel Big Trouble. Matt and Jenny fall in love throughout the events of the book, but when their parents get together, they decide it's just too weird and break it off, but remain close friends.
Played with in Augusten Burroughs' 'Sellevision' where two main characters fall in love and bang, then find out they are related but they're actually not... whew.
Granted, "small children" here is about ten, at least in Harry's case, which is more than old enough to avoid feeling like brother and sister.
Inverted in Orson Scott Card's Speaker for the Dead: Ender's speaking makes it public knowledge that Miro and Ouanda are, in fact, blood half-siblings. Given that the alternative was that Miro's dad's debilitating disease was profoundly different from every other case in recorded history in that it didn't start by obliterating his reproductive capabilities, you'd think somebody would have worked it out sooner, but the entire colony is profoundly Catholic and so the idea of his wife stepping out on him to have five or so kids just never crossed anyone's mind. It's pretty sad.
Almost no one else knew about the disease until the man's death. His wife hid the records.
In the Lord John Gray spinoffs of Diana Gabaldon's Outlander series, the main character finds out that... fairly unfortunately, his new stepbrother was a guy he used to sleep with. It's awkward, but that apparently doesn't last long, and they're promptly back together. Which is OK, except for the fact that they're living in the 18th century and hence have the appropriate awkwardness. (There was Character Development in there somewhere, but it still managed to squick some readers...).
P. D. James's crime novel Death In Holy Orders featured a half-brother and sister who discovered each other's existence as adults and start a casual sexual relationship. The sister, in particular, behaves as if it's no big deal, and makes the excuse that it's not like they're full siblings. Of course.
At the very end of Elizabeth Peters' book The Love Talker, main character Laurie learns that her brother was actually adopted. Based on some of his earlier actions, she concludes that he knows this and has romantic intentions toward her. She also immediately decides she likes the idea, saying something about how it's good to know, because she'd draw the line at Brother-Sister Incest.
Another Elizabeth Peters' example comes from her Amelia Peabody series: Amelia's son Walter "Ramses" Emerson is attracted to his adopted sister Nefret pretty much from the moment he meets her, but it takes Nefret a long time to see Ramses as anything other than an Annoying Younger Sibling.
Expiration Date: there is a passing reference to the protagonist's twin sister trying to force him into bed.
Last Call: the protagonist marries his foster-sister (at that point the full-fledged earthly representation of a syncretic moon goddess). Mythologically speaking, they can't really win the game (as it were) without marrying; the Squick factor is potentially mitigated (or greatly enhanced) by the fact that the protagonist was fifteen or sixteen when his sister was born and at the time of the novel hasn't seen her since she was a child.
The Stress Of Her Regard: after the death of his wife, the protagonist is Mistaken for Murderer by her Angsty Surviving Twin. She later comes to accept that he is innocent of the crime, and for years they work together (and pass for) brother and sister. When at a later point they have to pass as husband and wife, there are complications.
The Dark Elf Trilogy: Wulfgar and Catti-brie, both humans and both adopted children of the dwarf king Bruenor Battlehammer, are not otherwise related. Which I guess means their relationship is okay. It doesn't last anyway.
Odd subversion in The Firebringer Trilogy: When hero Jan's father Korr flies into a rage over Jan's marriage to the healer's daughter Tek, Jan doesn't understand, until he finds out that Tek's father was not the healer, but actually Korr. This in turn sends Jan into something of a Heroic BSOD, and when he sees Tek again and tells her what he has learned, they agree to renounce their marriage, believing themselves to be half siblings. Just before they actually do so, Jan's mother steps in and reveals that the pair aren't actually related by blood- Korr is not Jan's father after all.
Both played straight and subverted in One Hundred Years Of Solitude. Rebeca marries José Arcadio, as Rebeca technically isn't related to him, but their parents kick them out of the house anyway. The second example is a spoiler for the end: Amaranta Ursula (seventh generation) and Aureliano (the second) Babilonia get married under the pretense that they aren't related, but then Amaranta Ursula gives birth to a child with a pig's tail, he is eaten by ants, and Aureliano Babilonia realizes that Amaranta Ursula is his aunt.It Makes Sense in Context.
Jack Harkaway, a popular Penny Dreadful hero, fell in love and eventually married his adopted sister. They had a kid together.
There's a Mills and Boon medical romance called something like A Wife for the Baby Doctor where the eventual wife of the title is the hero's adopted sister.
Even though V. C. Andrews' works are best known for Brother-Sister Incest, it's actually more common for the heroine to end up with a man she initially believes is her brother/half-brother/cousin until it's revealed that they are not actually blood-related, making it fine for them to marry each other. The best example of this is the Cutler series where Dawn learns that she's actually adopted and that Jimmy is hence not really her brother; Jimmy even expresses his relief about this as he was attracted to Dawn even when he thought they were related. (And just to top this all off, Dawn's original boyfriend Philip turns out to be her actual brother who remains obsessed with her after the truth of their relations comes out to absolutely creepy levels. Got to love VC Andrews' wacky plots). Other examples are Heaven's daughter Annie and apparent half-brother Luke from the Casteel series, Melody and apparent cousin Cory from the Logan series (okay, it turns out they're still blood-related but only as second cousins, which makes marriage between them legal), and Rain and apparent brother Roy from the Hudson series (although they don't end up together, and there's that thing about Rain's own blood brother being attracted to her too - she doesn't end up with him either, thank goodness).
In the Harper Connelly series written by Charlaine Harris, Harper develops romantic and sexual relationship with her stepbrother Tolliver that eventually leads to them becoming engaged, much to the shock of their relatives.
Making it especially squicky, they have two mutual half-siblings from his father's marriage to her mother.
Georgia and Shaun, from Mira Grant's Newsflesh Trilogy. Heavily implied in Feed, the first book, and confirmed in Deadline, the second.
In Rafael Sabatini's Scaramouche, Andre-Louis and Aline are raised together, and Andre-Louis refers to her as a "sort of sister". They are completely in love.
A Song of Ice and Fire has this a lot, mostly due to the Westerosi custom of fostering their children. Romances between foster-siblings aren't considered anything out of the ordinary and are often encouraged:
Petyr Baelish played "kissing games" with both his foster-sisters, Catelyn and Lysa Tully. He eventually fought an (unsuccessful) duel for Catelyn's hand and later married Lysa.
Tyrion Lannister invokes this deliberately by sending his 9-year-old niece Myrcella Baratheon to live with her fiance, 11-year-old Trystane Martell, years before they will be old enough to marry.
Quentyn Martell mentions that there was a general expectation that he would marry his foster-sister, Gwynth Yronwood. He doesn't, but it would probably have been better for him if he had...
Not quite a straight example, but Robert Baratheon was supposed to marry Lyanna Stark, who was the younger sister of Robert's foster-brother Ned.
Another example can be found in Theon Greyjoy. Although they never interact in-series, he remarks several times that he'd hoped to marry his foster sister Sansa Stark, even though he knew it would never happen. Later, when he takes Winterfell, he thinks that, if she weren't in King's Landing, he might have forced her to marry him to secure his hold on the castle. Made somewhat squicky since, the last time he saw her, he was nineteen and she was eleven, and she's only two years older now.
A Villainous Incest couple who actually produced a child in A Brother's Price were considered siblings by the culture - marriages consist of one man, maybe two if they're lucky, and a band of sisters who are themselves descended from sisters, and all offspring from this marriage are considered to be siblings - but genetically are cousins. The social consideration overrode this trope and the couple were regarded with disgust and horror regardless of genes. The trope itself was used to point out that their child wasn't as poorly off as she could have been.
In the Hetty Feather series by Jacqueline Wilson, Hetty's most consistent love interest is her former foster brother Jem. In the final book, Emerald Star, he proposes to her and when she protests that she is his sister, he tries to convince her that they are allowed to love each other because they are not blood relations. Hetty rejects him anyway as she is leaving to join the circus.
In the young adult novel Cluing In by Jo Ramsey, main character Jamey Mandel's parents, Tamara and Sam, are step-siblings; Tamara's father Victor is married to Sam's mother Martha. Somewhat subverted in that Tamara got pregnant with Jamey (at age 17) before she and Sam realized their parents even knew each other, and before Martha got pregnant by Victor. Victor and Martha got married before Tamara and Sam did, though.
Stepsiblings Boone and Shannon in the first season of LOST.
Subverted for all it's worth on Arrested Development. Cousins George Michael and Maeby are attracted to each other and George Michael clings to any hope that Maeby isn't his Aunt Lindsay's daughter. When they find out that there's a chance she's been adopted, they go to second base... then they find out that she is Lindsay's daughter. As a result, Maeby begins dating a boy from school - Steve Holt. Meanwhile, Steve decides to find the father he never knew, who turns out to be GOB, Maeby's uncle. As a result, they break up. Then Maeby's mother Lindsay turns out to be adopted. Lindsay promptly starts hitting on George Michael's father, who is completely freaked out after thinking she was his twin for 37 years.
And then, "in a kneejerk act of revenge", Michael's brother GOB decides to hit on his adoptive sister Lindsay.
Lampshaded with the in-series movie Les Cousins Dangereux, a stereotypical french movie about cousins in love. When it's finally re-made for American audiences, the narrator explains that "the studio [...] had hoped to avoid controversy by artlessly explaining that the two leads were not biological cousins" but it didn't stop the protesters from picketing the premiere.
A callback joke has narrator Ron Howard remind the audience that George Michael and Maeby might not be actual cousins.
Despite the number of possible pairings and the age of the oldest kids when their (step) parents first met, not just subverted but entirely avoided in The Brady Bunch. It is, however, constantly mentioned in any parody of the show. It's also a subplot in the second Brady Bunch movie, when they think their parents didn't actually get married.
Likewise completely avoided in the ABC sitcom Step by Step. Though considering how... unBrady-like their relationships were amongst themselves, this is somewhat more understandable.
Not avoided in an episode of CSI: Miami, where the father of a murdered woman's baby is discovered to be her brother: turns out they met and became step-siblings when their parents married (and the two kids were 18 at the time) and one thing led to another.
Nate from Six Feet Under has an affair with his newly introduced step-sister. Nate is neither squicked by this fact nor the fact that he's screwing around on his wife with someone he's known for less than a month. This, plus Nate's trope-naming Karmic Smackdown had Nate's portrayer, Peter Krause, not happy at all.
On Two and a Half Men, Charlie becomes sexually involved with the daughter of his mother's fiancé, repeatedly pointing out that she will only be his step-sister.
Arthur and Morgana in Merlin. Morgana was raised as the ward of Arthur's father Uther. No expressed romantic interest on either side, but definite sparks.
Then in "The Crystal Cave," it is revealed that they are blood siblings! Though technically, that's been an It Was His Sled for several hundred years now.
Soap operas do this sometimes, particularly Days of Our Lives, where almost everybody is related to everyone else somehow (puzzling out the Brady/Horton/Kiriakis/DiMera family tree is so complicated that NBC even runs a commercial spot with star Ali Sweeney claiming she can't figure it out). Lately, Max Brady (adopted son of Caroline and Shawn, brother of Roman, Bo, Kayla, and Frankie) has dated two of his female nieces, Stephanie (daughter of Kayla) and Chelsea (daughter of Bo), and nobody has found this the slightest bit strange or squicky. Of such fine technicalities are the constantly-churning relationships of soap opera characters kept acceptable to the Moral Guardians.
Not canon, but this is the rationale people who 'ship Brooke and Sam of Popular together use. That and the fact that they were about 16 when their parents married.
On Battlestar Galactica (2003), Kara Thrace and Lee Adama are not biologically related, but they consider the same person to be their father (Bill Adama) and Kara was engaged to Lee's dead brother. Their interactions wildly vary between very close, occasionally juvenile best friends and a highly-charged sexual relationship.
Also any sexual relations between the seven models of humanoid Cylons, such as Boomer/Cavil and Caprica/D'Anna, as well as the failed attempts at Cylon breeding offscreen.
Adrian and her step brother Max in Secret Life of the American Teenager.
In the season two premiere of Reaper, Sock, once he gets over the shock of a stranger living in his mother's house, instantly lusts after his hot Japanese stepsister whom he just met when they were both adults, and did not grow up together. The problem is, she just wants a genuine sibling relationship between them, and is completely oblivious to his attraction (until it gets really, reallyAnvilicious).
On Dark Angel, Max and the rest of her X5 unit consider each other siblings because they were "raised" together. Zack falls in love with Max, and it Squicks her out. She wasn't too sanguine about the idea of being breeding partners with Alec, a clone of her brother Ben, either.
Dexter: Although Dexter and Deb are not in the least bit involved, there was an instance where when Deb got angry with Masuka for not inviting her to Dexter's bachelor party. He said that it was men only, unless she had wanted to be talent for the evening. Deb says that is gross as they're siblings, to which Masuka replies "not by blood." Dex just puts on his killing face in response.
Turns out Dexter may actually be Harry's bastard son, which would make Dexter and Deb half-siblings
No, Harry was just having an affair with Dexter's mom. Dexter's biological father, a bowler and ex-con, was confirmed through a DNA test in the first season.
Deb's own psychologist tries this excuse while suggesting that Debra's love for Dexter may be more than just sisterly.
Deb's canonically in love with Dexter now.
On Roseanne, Darlene and David's relationship didn't start this way, but it became more and more the case as the series went on. When David was "adopted" (taken in) by the Conners, Roseanne let him know he could move in by walking in on David and Darlene kissing and cheerfully crying to Dan "Oh, look, honey, our kids are necking!" By the time they got married, Jackie opened the episode by saying "I can't believe our little Darlene is marrying...our little David," Mark scolded David for asking about their birth parents when the Conners were practically his parents, and David and Darlene paid tribute to "our parents Dan and Roseanne Conner" in their wedding vows.
Another episode centers on David being creeped out that he had an erotic dream about Roseanne because he thinks of her as his mother.
In season 1, Veronica Mars finds out that her mother and the father of her ex-boyfriend Duncan Kane were High School Sweethearts who may have continued their relationship as an extra-marital affair... meaning that she and Duncan may or may not be half-siblings. This revelation leads her to pull her car over and puke on the side of the road. Luckily, the paternity test that her father has done proves that she and Duncan aren't related. A few months later, they resume their relationship.
Though according to Word Of God, Duncan was SUPPOSED to be Veronica's half-brother for real, but the network wouldn't let them put Actually-Blood-Siblings sex on the air. So it was originally meant to be a complete (and squicky) subversion of the trope.
Justin and Rebecca on Brothers and Sisters. When they met they believed they were half-siblings, but when she did a paternity test it turned out that they weren't, and then they became a couple.
In The Sarah Connor Chronicles, as far as the rest of the world is concerned, Cameron is John's sister. The sexual tension, by the way, is palpable.
A particularlysquicky example on the Soap OperaThe Bold and the Beautiful. In 2003, Ridge discovered that his father was not Eric, but Massimo. Meaning his half-sister, Bridget, wasn't his sister. He told Bridget and they were kissing shortly thereafter. That's bad enough on its own, but consider their history. Ridge had been in a on-off relationship with her mother for 15 years at that point. He also helped Brooke deliver Bridget in Big Bear ten years ago. He and Brooke had an affair, so they all believed Ridge was Bridget's father until she was 4. She was even named Bridget after Ridge. So Ridge made out with a woman who he not only thought was his sister but also his daughter. Thankfully, any Ridge/Bridget pairing didn't get too far.
More than that, in the live-action Ninja Turtles: The Next Mutation it was claimed that none of the turtles were actually related to each other; they just happened to be the same relative age in the same terrarium at the time. This was done so that, when a female turtle shows up later, there would be the potential for a relationship. Buuut, we don't talk about her...
Subverted in That '70s Show, in an episode where Eric's hot cousin comes to visit and tells him she's adopted, and then seduces him just to trap him into making a move on her in front of his parents. She's not adopted at all; she just wanted to pay him back for a prank he pulled when they were younger.
Subverted in the 2009 TVB drama Born Rich: The family's "long-lost" half-brother is actually dead and the "brother" that they have been interacting with is actually a twisted con man who stole the dead man's identity and wormed his way in. He develops a romantic obsession with his "sister" to the point that he rapes her. Needless to say, when she discovers the truth about her "brother"'s identity and his actions, her previously conflicted feelings turn into hatred and revenge.
In Wonderfalls after Mahandra and Aaron sleep together she freaks out because they grew up together, although they're not actually adopted siblings.
Played With on 3rd Rock from the Sun when the Solomons attend a family reunion pretending to be long-lost relatives. Tommy falls for one of the other guests, but she freaks out because he's supposedly her cousin. Harry, meanwhile, has been catching up on family gossip and reveals that she was adopted, which Tommy expects will fix things. It doesn't.
Smallville featured an example on the alternate-universe Earth Two: Tess Mercer is having sex with her adopted brother, Clark Luthor. When you think about it, the boy was adopted, and the girl was never taken care of by her father. So even if both have Lex as a brother in one way, they are hardly siblings in any way.
Perhaps the most sick and overly complicated example imaginable on Passions. Chad who is biracial comes to town and eventually dates Whitney the son of Eve and TC (the only black family in town). Chad is for several years hinted to be the son of Eve and her white lover Julian from before she was married. Later Chad and Whitney have a baby together and the revelation comes out that they are half-brother and half-sister and their baby was born from incest. Later is revealed that Chad is not really Eve and Julian's long lost son. It turns out Julian's father Alistair had raped Eve's adopted sister so yay Chad's son is not a product of incest but on the other hand Chad is a product of rape. Even later it is revealed that Chad is having a gay relationship on the down low with a guy named Vincent...who turns out to be Eve and Julian's hermaphroditic long lost child for realsies this time...oh and he's a serial killer and a rapist to boot. So yeah Chad didn't sleep with his half-sister...but he did sleep with his girlfriend's half-brother who was also his own half-nephew.
A very strange example in 16 and Pregnant (and spin-off Teen Mom): couple Catelynn and Tyler are technically step-siblings, because Catelynn's mom April and Tyler's father Butch are married. Butch and April met and married after their childen started dating.
In Vocaloid, we have Rin and Len Kagamine, who fans are debating over whether or not they are twins... despite the fact they have the SAME. FREAKING. LAST. NAME. Some claim that they are merely only genderbents of each other to avoid having their fanfictions labeled as Twincest, while others are eager to embrace the twincestiness that comes with them.
To a lesser extent is the genderbents of the vocaloids (i.e. Mikuo, Kaiko, Gakuko, Luki, Ted); the fandom can get away with pairing them up because they are simply gender reversals of the official vocaloids and therefore are not canon siblings.
In an attempt to placate the detractors, the creators declared that Rin and Len were literally mirror-versions of each other, so probably somewhere between Half-Identical Twins and Opposite-Sex Clone. Of course, considering that all the Vocaloids are computer programs...
Fans of Baldur's Gate frequently debate whether a relationship between the PC and Imoen, both Children of the dead god Bhaal, would "count" as incest. Those who favor the relationship usually point out that the PC is often a different race from her, and that canonically they are known to not have the same mother, so that their main connection is definitely not any mortal source. (This is even brought up in the expansion Throne of Bhaal in a discussion between her and one of the other party members; Viconia suggests she should serve the main character in every way he desires for all that he's done for her, and why it shouldn't be a problem. Imoen balks at the thought and finds the idea disgusting... and then turns the question back on Viconia, who's also been canonically saved twice by the protagonist and is a romance option). Ultimately irrelevant, as she isn't a romantic option in the game... unless you mod it. And the only mod currently available and semi-complete is not very good.
The same argument is made for the female PC and Sarevok, who are also Children of Bhaal from different mothers. It's a little easier to defend, since Throne of Bhaal makes it clear that Sarevok no longer has the Essence of Bhaal, and that was how he was related to her. Once again, it's irrelevant since he's not a vanilla romance option. Though his mod romance is better than Imoen's, at least.
In the Mega Man Legends universe, Rock (Mega Man) is literally Roll's adopted brother, yet romantic hints exist. Fans debate whether the original series Rock (Mega Man) having a relationship with Roll would count as incest, as their original incarnations are both robots and no genetics are involved, despite being "siblings" created by the same man. Mega Man NT Warrior Rock and Roll have no familial relation at all.
Tales Of Legendia, in which Senel and Shirley pretend to be brother and sister (arguably, they do rather act as though they are, but they certainly don't resemble the other in the slightest, and have different surnames, to boot) until they are exposed by the main villain as not being related at all — in fact, each of them belongs to a different race altogether. Not too long after this, Shirley confesses to Senel that she is in love with him. They presumably end up engaged via the Rite of Feriyen by the end of the Main Quest ... although afterward, the other characters note that the relationship between them doesn't seem to have changed much at all (they even get new titles to reflect this; Senel obtains "Still A Brother" and Shirley obtains "Sister Girl").
In fact, Selphie and Irvine are the only ones who actually remain inside their raised-together group for romance. Everyone else from the orphanage looks outside the group for romance, as Zell hooks up with Library Girl and Quistis... uh, gets shafted, pretty much.
To be fair, she does have a fan club. And boyfriends would have to face the whip.
Final Fantasy II mostly contains only hints of love interests but the No Export for You novel does have Maria harboring feelings for her adopted brother and the story's main character, Firion.
The plot for the Kingdom of Caerleon in Brigandine involves King Cai falling for his sister, who turns out to have been adopted.
The Fire Emblem series tends to prefer the full-blooded version of this trope, but apparently decided to go the Not Blood Siblings route in Radiant Dawn... twice. Once with Micaiah and Sothe, and also with Elincia and Geoffrey. In both cases, they end up married at the end if they have an A-support with each other.
Micaiah and Sothe is a combination of this and reversed Wife Husbandry. Due to her long life span she raised Sothe from when he was a kid, though it could be argue that she viewed him as a little brother since physically she is only in her mid to late teens. Interestingly, they are the only characters who start out with A support for each other.
Not the first time the Fire Emblem series has done this, though. Near the end of the first half of Genealogy of the Holy War, King Trabant of Thracia kills Cuan and Ethlin and takes their daughter, Altenna, to raise as his own. She ends up falling in love with Trabant's biological son Areone.
Emma Emmerich, in Metal Gear Solid 2, has a thing for her big brother Hal "Otacon" Emmerich. Turns out that they're really just stepsiblings; they have the same last name because Hal's dad adopted Emma after he married her mother. The sexual relationship which developed between Hal and Emma's mother, however, adds an extra layer of Squick to the situation.
In the game Lunar Luna is Alex's adopted sister, and the two of them grew up together. Yet no one in the entire game, even Alex's parents, comments about their romance.
A similar non-Squick instance for Joshua Radcliffe and Cliana Rimskaya of Super Robot Wars Destiny: Cliana was adopted by Joshua's father and throughout the game, the two treat each other Like Brother and Sister, except Joshua is sometimes called out for his Big Brother Instinct by insisting Cliana stay away from the battlefield. Then again, when your adoptive sibling has a slightly, unstable Split Personality, you'd probably do what Joshua does.
Vandal Hearts 2: In a Bittersweet Ending (by letting your childhood friends die), you make good on the promise you made to Clive to take care of Rosaly, your adopted sister, and the ending is that she gave birth to your child. In a semi-justification, Joshua (main character) was adopted when he was almost to his teens, and was separated from Rosaly for a long time over the course of the plot.
Hiroki and Elis in Canvas 2. She's actually his cousin, but she's stuck very firmly in the 'little sister' category when thinking of love interests. He writes it off as a Precocious Crush or her just joking around despite her being persistent about it to Single-Target Sexuality levels.
The canonical routes in both Da Capo games have always been about Not Blood Siblings, though the second game strayed into a fine line between this and Kissing Cousins for both Asakura sisters, since the protagonist is both adopted and raised by another parent - who, in turn, is another Not Blood Siblings with Asakura grandpa.
Which makes his surrogate son - Yoshiyuki - the adopted distant uncle of the Asakura sisters, which make his relationship with either sister one generation apart... No wait, that't not right. Hmm... Try to wrap your head around the family tree of this franchise.
Yuri and Kira from Infinite Space insist that they are siblings while everyone around them think they're just very close friends who have a thing for each other. Yuri even gets someone to do DNA tests that reveal they have no blood relationship. They're Artificial Humans created by extra-dimensional aliens and they're only "siblings" because that's the first relationship that popped into Yuri's head at the beginning. A previous scene established that he had no relatives at all to leave behind when he went into space.
The curate and his [adoptive] sister in the online game Echo Bazaar. It is possible for you to seek romance with either (or both) of them, and during the process you can find what looks like a fragment of a love letter from the sister to him.
Claimed in the penultimate route of Duel Savior Destiny, but then undone in the actual final route. The story first claims that it's so convenient that Taiga and Mia aren't related in the resolution to her route, but in the Crea/harem route that follows it the story notes that they have a blood relation. Whether this is a subversionnote Meaning they were simply wrong in the previous exchange about the matter or a simple retcon is unclear.
In the Visual NovelCrescendo, this is also an important plot point. Ryo is living with his adopted sister, (their parents having died three years earlier) when his birth mother asks him to move in with her family. How the player answers the question is a major branch point in the story.
Although it's pretty much one-sided, in Fate/stay night there's Shirou Emiya and Ilyasviel von Einzbern. She is the biological daughter of his adoptive father, and has an obsession with him that is at least partially romantic. The fact that Word Of God has openly stated that she was nearly the fourth romanceable heroine of the game and there are two separate Bad Ends where she magically makes him her slave for life (complete with heavy implications that he is 'that kind' of slave) really doesn't help.
Akane and Yoshikazu in Suika. Definitely doesn't go the usual way this trope normally does. You are given the choice to supposedly go that way, but then comes The Reveal...
An integral part of the plot of some arcs in the Tsukihime game. Shiki and Akiha met when they were seven and six years old, respectively, lived together for two years, and then separated for eight, somewhat short of "growing up together." Akiha knew the whole time that Shiki wasn't her real brother, and Shiki was at least vaguely aware of it, even admitting to himself once that his protective instinct towards her didn't really seem like sibling love. Needless to say, that really only comes into play during Akiha's arc.
The fact that they are not related is important in all the arcs due to the events that occurred ten years with SHIKI, the real brother of Akiha.
Konstantin seems to think this is going on between Onii-chan and Kotone in Experimental Comic Kotone. It isn't. What none of the characters except Kotone-chan herself and Onii-chan know is that Kotone is a Robot Girl — the whole "little sister" thing is just a cover.
The heads of the GameFAQs group "The yusketeers" Yusiko and Yukito can't have a conversation without sexual tension showing up somewhere, apparently people have a venn diagram just to tell them apart.
Some say that the sheer sexual tension that permeates any room the two are in is enough to stun a horse.
Devin, he guy who owns eFukt.com, says he's in a sexual relationship with his sister. He always adds that she's adopted, so it's okay. Here's what she looks like: ◊. Can you really blame him?
Zeus and Hera in Thalia's Musings. The Titans are beings of pure spirit who paired off and joined their life force to create the first generation of Olympian gods. Zeus and Hera, as well as Hestia, Demeter, Hades, and Poseidon, were created by Cronus and Rhea.
Athena and Artemis. Artemis is Zeus' biological daughter, and Athena is his ward and ex nihilo creation.
As the quote above demonstrates, Friendship Is Witchcraft's Twilight Sparkle justifies having a crush on her brother Francis (that universe's version of Shining Armor) by pointing out that Francis was a Doorstop Baby left at Twilight's house. Even after her explanation, the other ponies still find the attraction creepy. Twilight wins in the end, and eventually marries Francis.
On Tiny Toon Adventures, if Babs Bunny and Buster Bunny didn't always say "No relation" after introducing themselves, most viewers probably would have assumed they were siblings.
An odd example: in one episode of The Simpsons, Homer's father briefly dates Marge's mother. Homer is against their relationship, since he believes this will make him and Marge siblings, and retroactively turn the kids into freaks "with pink skin, no overbite, and five fingers on each hand!" The two wind up breaking up, though.
However, Marge later becomes Homer's aunt when Abe briefly marries Selma. Which is even weirder when you take his relationship with Mrs. Bouvier into account...
On Gargoyles, the title species lives in large clans that are considered to be a single family — nobody keeps track of who their biological parents are, and thus all gargoyles of the same age group refer to each other as their "rookery brothers/sisters" and older members as their "rookery parents." Word Of God says that real incest never occurs, though, because a) each female can only lay one egg per mating cycle and b) pheromones prevent Kissing Cousin relationships.
On Ugly Americans,Mark and Callie raise the latter's younger sister, Lillith (which admitted only took about two weeks). Lillith then declared her intention to kill Callie and have sex with Mark in front of a stadium full of other demons. Mark, who had spent the entire episode acting like a doting, enthusiastic father, is understandably freaked out.
Roger: Everything that happens from this point is just gravy.
At present, Japanese law allows for adopted siblings to marry, which has feudal-era origins. Japanese family inheritance traditionally emphasize the transmission of family traditions rather than blood, so at the time titles can only be transmitted to the title-holder biological or adopted son; even nephews need to be adopted as an heir. This caused adoptions in feudal Japan to be as political as Arranged Marriage, and many just do both— adopting a child from another family as one's heir and have the latter marry his own daughter.
As an example: Matsudaira Katamori, ninth and the last Daimyo of Aizu and the direct boss of Shinsengumi, is an example of this andKissing Cousins. He was born an inferior son of the tenth Daimyo of Takasu, whose biological brother (i.e. Katamori's uncle) was already adopted out and became the eighth Daimyo of Aizu. When the latter, again, was facing the problem of a lack of male biological issue, he adopted Katamori, his biological nephew, as his son and by extension, the future ninth Daimyo of Aizu. To cement the relationship between the two houses, the eighth Daimyo of Aizu arranged Katamori to marry his biological daughter, i.e. Katamori's first cousin by blood and sister by adoption.
The Roman emperor Tiberius married his step-sister Julia. Given that Julias father Augustus and Tiberius mother had already been together when Julia was born to Augustus first wife and Tiberius was already 14 at that time, it's still more than a bit squicky. But to their credit it wasn't their idea and and didn't last long.
A variant of this can happen with known sperm donors (when a women/couple use a friend or someone they know to conceive a child due to being unable to do so on their own). It's not uncommon for the sperm donor to stay involved in the child's life afterwards. This can be a little confusing when the child finally asks who the father is and learns that it is 'uncle' Drew. Often they child will continue to refer to the sperm donor as Uncle in this case, since he is not the parent that has raised and cared for them. This can be pretty natural to the child, who often learns this fact at an early enough age to take it for granted (in the case of single and lesbian women it's pretty clear even to a young child that someone else was involved), but it can come as quite a shock or even a bit squicky to outsiders trying to figure out the relationship of the 'uncle' to the child. It can be far more confusing when a known donor was used for a heterosexual couple with fertility issues, as the couple may choose to not tell the child that a donor was used till they are much older which can make the revelation much more of a shock for the child.
A common practice for lesbian couples is to ask the brother or father of the non-carrying partner to donate, so that the child shares some of the partner's DNA. In this case the father really IS the uncle, just not genetically.