Peter Brady: So these feelings are natural?
So you've got two unrelated teenagers whose parents have recently gotten hitched. Said teenagers happen to have compatible sexual/romantic orientations. Whether they find it squicky 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 just come off as flirty.
The Romance Novel 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 parent as well).
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. This may also cross the line in the supposed Westermarck Effect. A theory that 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 Truth in Television for many stepsiblings whose parents married during or after their puberty, though they rarely end up becoming an official couple due to social factors. However much actual people find this trope, well let's just say Incest Is Relative.
- The manga Yoru Made Matenai is built around this trope, with a wee bit of borderline shotacon fanservice.
- The main couple of Marmalade Boy, which starts with the two leads's parents switching partners and then all deciding to live together in one Big Fancy House.
- The whole point of Akuma de Sourou. 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 Magical Girl Lyrical Nanoha Strikers 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 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: Mion has fallen in love with her classmate Tsukasa, but 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 share Tsukasa with her big sister…
- kiss×sis is about two twin sisters who love their stepbrother very much. Their mom and the stepbrother's dad approve. He doesn't. Hilarity Ensues.
- Another rated manga example, Ai Don't Koi from Yasuhira Nakanishi. The main character struggles between two choices: one is his Unlucky Childhood Friend 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 father.
- Yuuka and Atsuro Kiryuu in Sensual Phrase, though only in the manga. One of the side-stories focuses on this, and specially when Riri Kinoshita, an Idol Singer 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. And then she pisses him off via verbally abusing poor Yuuka...
- Cherry Juice: As the series progresses, step-siblings Minami and Otome often switch between sharing flirtatious romantic moments and Belligerent Sexual Tension.
- Subverted in a case of Detective Conan. A woman named Kimie had to give her toddler daughter Yukiko away to her husband's family due to 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 Freak Out! and forbids Takayoshi from dating Yukiko, having recognized the girl's identity as his wife's estranged child... which then leads to a horrifying reveal.
- Kazuma and Tsubasa from Kare Kano.
- In The Testament of Sister New Devil, predictably, Basara has much romantic and sexual tension with his new sisters Mio and Maria. But in reality, they are actually the new demon lord and her assistant respectively. But even after finding out, he still does what he can to protect them from demons and others that want Mio dead. And Maria, being a succubus, naturally likes to tease him.
- Yuzu and Mei from Citrus are two step-sisters who have to deal with their attraction to each other. Yuzu only liked males until she met Mei.
- Subverted in the manga version of Bunny Drop. One of the reasons why Yukari and Daikichi decided against becoming an Official Couple was to avoid this trope. Daikichi's adopted daughter Rin and Yukari's son Kouki have romantic feelings for each other as teenagers. Ultimately Rin and Kouki break up and Rin ends up with Daikichi. The anime adaptation and film adaptation ignore the timeskip so they have more implications that Daikichi and Yukari ended up together.
- Cruel Intentions ramps this trope Up to Eleven, with the plot driven by a girl promising her besotted stepbrother sex if he does what she wants. Not Another Teen Movie parodies this with them actually being related, and the brother being utterly repulsed at the idea, but going along with The Bet for other reasons.
- Clueless has Cher and Josh. It helps that their parents are no longer married.
- A Very Brady Sequel 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 — the two teenagers have a mostly-normal romance, and their parents get married at the end of the movie.
- The horror movie 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 Lone Star State of Mind (produced by Edward Zwick, of Glory and My So-Called Life 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 Jack Nicholson) points out that, since he is just as much in love with Daisy (played by Barbra Streisand) 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 The Perks of Being a Wallflower. 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're step siblings. Not only that, but Patrick is gay as well.
- In Lovers Of The Arctic Circle, 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 Mr. Nobody started off as Childhood Friends who later become step-siblings and start developing a romantic relationship.
- Blair and Aaron in the Gossip Girl books.
- In 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. In the movie, Jessica calls attention to how weird it is for them to act as siblings and be in a relationship.
- Julia and Tiberius in Jeg elsket Tiberius (I Loved Tiberius).
- Harry and Elaine from The Dresden Files. They were both adopted by their Evil Mentor in their early adolescence, having no known genetic connection. Once puberty struck they took their relationship in the expected direction. 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 Amelia Peabody 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.
- In the 18th-century French Fairy Tale novella Princess Lionette and Prince Coquerico, the protagonists, Lionette and Coquerico, are stepsiblings. About midway through the story, it's revealed that Coquerico's mother, after handing over her son to the fairy Cornue, met Lionette's father and married him. (And, being a fairy tale, Wicked Stepmother comes into play, with Coquerico's mother devising the plan to get rid of Lionette.) This revelation does not prevent Lionette and Coquerico from falling madly in love with each other—the two of them get married at the end of the story.
- Malevil has Jacquet and Miette. They are lovers as well, but Jacquet has to share her with every other man at Malevil.
- A technical version occurs in Song of the Lioness, when Alanna's 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, Wuthering Heights 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 Brothers Conflict, specially its visual novel counterpart.
- Discussed at length in We Can't Rewind:
Denise: "What if all this other stuff had never happened? I mean, what if we'd never gone on this cruise, just spent a week in Vegas for our honeymoon, or something, and then we'd gone home and raised our kids normally? I mean, as normally as we could, of course. And what if they'd just naturally gotten sweet on each other when they got to be teenagers? Would you have let them go on dates with each other?"
- Derek and Casey from Life with Derek have a relationship full of Belligerent Sexual Tension. This was unintentional on the writers' part, but very intentional on the actors'.
- Lost: Boone is in love with his step-sister Shannon. Shannon is aware of it and frequently uses it to take advantage of him, culminating in them having sex followed by her blowing him off once she's gotten what she wanted.
- The Secret Life of the American Teenager
- Reaper has Sock getting a stepsister. She thinks of them Like Brother and Sister, but tends toward much I Didn't Mean to Turn You On. He's trying to get in her pants. (He eventually succeeds.)
- Los Serrano has the relationship in between Marcos and Eva. Lampshaded in a dream sequence ( despite the series itself being All Just a Dream, but anyway) where Marcos says their relationship isn't incest, "more like stepincest".
- From the Taiwanese drama Devil Beside You, Qi Yue was being bullied by the son of the school's president, Jiang Meng, only to fall in love with him, after realizing that he was a Jerk with a Heart of Gold 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.
- Morgana and Arthur in Merlin had a very charged relationship in Season 1. "The Crystal Cave" reveals that Arthur's father Uther is Morgana's biological father, having had an affair with Gorlois' wife Vivienne.
- Inverted by Gina and Boyle in Brooklyn Nine-Nine, they had a sexual relationship, but when their parents got together and eventually married they had already broken up.
- Glee: Now that their parents are married, Kurt and Finn. Of course, Kurt originally tried to get Burt and Carole together in an attempt to invoke this, as he had a crush on Finn for months beforehand (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.
- The Fresh Prince of Bel-Air: 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 "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. At the end of the third season the two are a couple and Lena is pregnant.
- On 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.
- Modern Family has a Flirty Step-Uncle. 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.
- Shake It Up! has the Belligerent Sexual Tension variety between CeCe and Logan , although it is subverted since the parents didn't get married.
- In Gossip Girl, Serena and Dan find themselves in this situation when their parents, Lily and Rufus, marry each other. After they divorce, Dan and Serena get together, and end up marrying in the finale.
- Pretty Little Liars: Stepsiblings Jenna Marshall and Toby Cavanaugh.
Aria: Toby and Jenna had a relationship.
Spencer: Way beyond step-siblings.
Aria: Yeah, they… were involved.
Spencer: I doubt I'd call it romantic—I doubt she had much of a choice.
Jenna: You need to be home. Where you're safe.
- Because All Abusers Are Male and Double Standard: Rape, Female on Male, it's assumed that Toby was the perpetrator. He wasn't.
Toby: You could chain me to this porch and I'd still never touch you like that again.
- Justin and Tiffany are this in Waterloo Road. The end up secretly dating.
- Michael Fairgate and Paige Matheson had a relationship in Season Eight of Knots Landing.
- Valkyria Chronicles: The main character makes his "Adopted" sister's non-blood relation clear to a very suspect degree.
- Master Li in Jade Empire runs a martial arts school, and his two prize students (Dawn Star and the Spirit Monk) both are his adopted children. After certain revelations about Master Li, it becomes apparent that there is also a case of flirty stepcousins if the Spirit Monk enters into a relationship with Silk Fox.
- In Fire Emblem Fates, any marriage between the Avatar and their Hoshido "siblings" is revealed to be this, and not what they originally thought. Additionally, the Avatar and their Nohr siblings are this as well.
- Also, Azura can marry her Nohrian step brothers, Xander and Leo. Arguably, Ryoma and Takumi could count, too, since she was never officially adopted but lived with them for as long as the Avatar was in Nohr.
- The very last strip of Ozy and Millie, with a few hints shortly prior, indicates this is going to be how things are for the title characters.
- Inverted in Something*Positive. As part of her back story, Vanessa had a brief fling with a guy she knew from school, though the two broke up on amiable terms. Later, her mom married his dad. The awkwardness got significantly worse when the guy, who had a very laidback relationship with his father, casually brings this up, not realizing how much it would freak Vanessa's mother out.
- Mickey and Audrey on ''Home Schooled. The two usually spend their time bickering, so this could be Belligerent Sexual Tension / Slap-Slap-Kiss.
- Friendship is Witchcraft features a (mostly) one-sided flirtation between Twilight Sparkle and Shining "Francis" Armor. In canon, they're biological siblings, but in this universe, "They're not biologically related, so technically it's okay."
- 6teen has Jonesy and Jen in spades; they even kiss and Jen admits she used to have a crush on him.
- Teenage Mutant Ninja Turtles (2012) has Leonardo and Karai. They're flirty before they realize that Karai is actually the long-lost daughter of Splinter (aka Hamato Yoshi), who raised Leonardo from childhood. After the reveal, they actually grow closer, and though the attraction isn't explicitly referred to as much, they tend to treat each other a bit too intimately for step-siblings.
- Martin Mystery: It takes the subtext to the nth degree. They were really pushing it to the limit. That's what happens when you take characters who were lovers in the original comic and turn them into Not Blood Siblings.
- The amount of Ho Yay between Gumball and Darwin in The Amazing World of Gumball is...well, amazing. Especially in "The Bros", when Darwin gets jealous of Gumball's actual girlfriend. They're actual adopted siblings rather than stepsiblings (Darwin used to be the family's pet), though this arguably makes it worse because they've lived together since Darwin was two and Gumball was four.
- In one episode of The Simpsons, Homer is opposed to the thought of his dad marrying Marge's mom.