Warning: Spoilers Ahead.
Relatives getting mistaken for Love Interests in Fan Works.
Crossovers
- In Avenger Goddess, while Diana isn't a biological relative of Howard Stark or Natasha, she considers them her family to the point that she is disgusted and outraged at the implication that she might have been romantically involved with either. When a teenage Tony accuses her of being Howard's mistress after Howard's funeral, she is outraged and disturbed at the idea that she had that kind of relationship with Howard when she actually saw him as a brother, and when Clint first meets Diana and Natasha, he initially wonders if they're dating and is surprised at the intensity of disgust they express at that idea (Diana is actually Natasha's adopted mother).
- In the Buffy the Vampire Slayer/NCIS fanfic Little Brother
, Xander Harris answers the door of his hotel room to find Gibbs and Tony waiting for him, wanting to ask if he knows anything about a murder in the same hotel the night before. When they hear their teammate Abby in the same room, sounding as if she's in pain (she had just woken up with a hangover), they flip out and nearly arrest Xander on the spot. It takes a few minutes until Abby herself sees this, and her angry reaction is "You cuffed my little brother?" - Humorously subverted in the Buffy the Vampire Slayer/Supernatural fanfic Runs in the Family, where Xander asks Sam and Dean if they're brothers, which Dean interrupts with "Dude! We are NOT gay!" Given the number of times they've been asked if they're together in the actual series, his reaction is understandable.
- In Hero High, many people think that Ben and Gwen are a couple. Though it doesn't help that the author enjoys adding the Ship Tease.
- Of Monsters and Men: Cornelia thinks this when she hears that her sister kissed a strange man. She doesn't realize that the man is their long-thought dead half-brother Lelouch and that the kiss was purely platonic.
- Inverted in the Frozen fanfic Never Been Bothered, where the protagonist mistakes Anna and Elsa for being sisters instead of a couple. Later subverted when it turns out they are siblings. They fled their kingdom, along with Kristoff, Sven, and Olaf, to be together.
- In one of doodlegirll's stories, Wilbur Robinson's girlfriend catches him embracing a pretty girl and runs off in tears, assuming he's cheating on her while the readers know said pretty girl is Wilbur's daughter with said girlfriend, back in time to hang out. Of course, Fridge Logic sets in when Wilbur doesn't tell her this since his girlfriend is fully aware that time travel exists.
- In Dragon Ball Z Abridged, Bulma ends up straight-up propositioning Trunks, unaware that he's her Kid from the Future. Once she discovers the truth three years later, thanks to Piccolo, it takes a while to realize what she tried to do. When he gets back to his own time, he brings it up with his mother, and she's perfectly understanding of her alternate younger self:Future Bulma: Well, duh!
Future Trunks: MOM!
Future Bulma: Hey, it's not my fault your dad's genetics and mine got along like chocolate and peanut butter.
Future Trunks: Is that why you used to call me your "little peanut butter cup"?- In Episode 49, the islanders believe Cell and 18 are a couple that has serious relationship issues rather than brother and sister, respectively, though it doesn't help that Cell's behavior towards 18 is eerily similar to a Stalker with a Crush.
- In Episode 10 of the Dragon Ball Z Abridged spin-off Sequel Series, HFIL, Freeza and Cell leave the dinner table due to them being unable to stand the "prick" they're sitting with, Cooler for Freeza and Bojack for Cell respectively. After they both clarify who they were talking about, Cell briefly is confused at who Cooler is, guessing that he's Freeza's ex-lover. After being told that Cooler is Freeza's brother, Cell simply says he "doesn't judge." (Which itself is a Call-Back to when he says he's not sure if Goz and Mez are 'brothers, or lovers, or both.')
- In Poisoning Apple, Raven and her older brother Rotbart don't look anything alike and no one actually knows that they share the same mother. Apple gets the impression that Rotbart and Raven are romantically involved to explain why Rotbart took Raven's place as Snow White's villain and why Raven is affectionate to him and spends time with him. This is seemingly confirmed when Rotbart gives Raven the very romantic gift of his mother's jewelrynote for her birthday.
- Fate/Harem Antics: Rin Tohsaka thinks that Caren Hortensia is Kirei Kotomine's lover instead of his daughter. Since Caren hates Kirei, she has no problem playing it up to make him look like a Pedophile Priest.
- At one point in Fate/Noble Shade, the protagonist sees Rin and Sakura interacting and assumes they're secretly girlfriends. Much later, he becomes privy to the fact that they're secretly sisters.
- In the Fate/Grand Order fan comic How Complicated Can This Family Tree Get, Artoria finds Lostbelt Morgan and Lancelot and accuses him of making a move on her sister. Lancelot protests that, one, he wasn't making a move on anyone, and two, that he was speaking to his adoptive mother Nimue, much to his king's confusion. Morgan then says that the Proper Human History versions of Morgan and Nimue are both the same person, specifically split personalities. Lancelot then leaves after realizing that the king he secretly held impure thoughts towards (including getting her older self into a bunny suit) was actually his adoptive aunt.
- A variant occurs in Hey, Asshole!. While they're not related in any official capacity, Zack sees Cloud as his younger brother and becomes very grossed out upon Sephiroth wondering if their relationship isn't completely platonic.Sephiroth: Were you romantically involved?
Zack: Were we- NO! Gross, dude, he's like my little brother! And he's fourteen! Eew!
- A vendor in My Master Ed assumes Edward and Hohenheim are brothers. Hohenheim is under the impression that their similar appearance is a coincidence and they aren't related at all. Only Edward himself knows they're father and son.
- In Returning Echoes, a jealous Winry slaps Alphonse and runs off when he brings home a blond woman, unaware said woman is Edward.
- In The Awakening of a Magus, Sirius gets angry when he sees Adelaide talking to Severus, only for the latter to point out that first, they are cousins (albeit distant ones), second, they were also good friends, and third, that who is Sirius to talk after breaking Adelaide's heart while at school.
- A Thing of Vikings: When Hiccup sees the way Toothless interacts with the Night Fury that has been attacking Mildew's village, he initially assumes that she is Toothless's new girlfriend, before Toothless uses Tuffnut and Ruffnut to 'explain' that the new Night Fury is his sister.
- A Song of Grumbling & Cotton Candy: Pizzazz mistakes Blaze's voice for Video's and accuses Clash of dating Video. Video and Clash are cousins.
- This Avengers High School AU, where Thor apparently forgot to tell all his friends he has a younger brother. Doesn't help that Loki is clearly not blood-related.
- In Blessed, Lila Rossi tries to maliciously slander Marinette and Nathaniel by claiming she witnessed Nathaniel threatening Marinette in the bathroom, forcing her to stay with him. This immediately makes the other students realize she's a massive liar — unlike her, all of them know that Marinette and Nathaniel are fraternal twins.
- In Burning Bridges, Building Confidence, Cole is a cousin of Marinette's who moves back to Paris and transfers to her school. Their classmates aren't aware of their relationship at first, and when Adrien tries to convince Marinette to stop trying to expose Lila's Malicious Slander, he refers to Cole as her 'girlfriend'.
- In Miraculous: Tales of Scarlet Beetle & Ikati Black, Kim was disturbed finding out that the seemingly older sculptor Théo Barbot had a crush on his superhero identity Scarlet Beetle. So when he sees Théo at his school and hugging his schoolmate Mireille, he freaks out and is about storm over to protect her until he sees Mireille hug him back in a familiar way. Kim then learns from Ivan that Mireille and Théo are half-siblings, and that he's from an art collège (a middle school) a few blocks from their own school.
- At the beginning of Swamp Call, Kermit's sister Maggie comes to visit from the swamp. Not knowing who she is, Piggy is jealous, and Maggie, being The Prankster, teases her by pretending to really be Kermit's girlfriend, until Kermit arrives and reveals the truth.
- In Juxtapose, Ochako thinks Toshinori is Izuku's step-dad because of how close they are. Hitoshi decides to perpetuate this misunderstanding to laugh at Izuku's flustered reaction. Later on, Izuku admits he sees Toshinori as the father figure he never had.
My Little Pony: Friendship Is Magic
- A Diplomatic Visit: In chapter 20, when Twilight refers to "Shiny", Maxilla teases her about it, saying "Getting a little familiar with your fellow princess's husband there, aren't you?". A rather indignant Twilight promptly informs Maxilla that Shining Armor is her older brother.
- In The Lunar Rebellion, Nimbus Kicker thinks her lover Radiant Day is cheating on her with his squire White Knight. Turns out, White Knight is his sister. And also his squire. But most importantly of all, his sister.
- In Over The Edge And Through The Wood, Troy assumes Applejack and Big Macintosh are Applebloom's parents instead of her siblings.
- Pony POV Series: The Cutie Mark Crusaders thought Prince Blueblood was Princess Cadence's ex-boyfriend. They are cousins.
- Quiververse:
- In story #15 (Scars of the Quill), when Flash Sentry arrives in town, he's greeted by Twilight, Quiver Quill and Sour Sweet, the latter of whom he went through guard training with. He promptly teases Quiver by saying "Now please tell me you and Sour aren't dating." Quiver quickly informs him that they're cousins.
- In story #17 (Many Happy Reunions), this happens to Quiver and Sour again. Sunrunner, Quiver's old classmate (and who has feelings for him), arrives at his house and finds Sour Sweet, whom she knows from their time in the Guard together, answering the door. Grimly, she asks why Sour is there, and Sour immediately replies that she lives there, then demands to know "why you took so damn long getting back in touch with my cousin". Sunrunner is quite visibly surprised, but also clearly relieved, to hear that they're related rather than dating.
- Six Brides For Two Sisters: Combined with Mistaken for Pedophile when Princess Cadence and Shining Armor recall how they first met: she saw him walking away with the filly she was foalsitting with promises of fun and candy, but since Twilight and Shining don't have any family resemblance (despite being siblings), she jumped to conclusions. Violently.
- One fan comic has Boa Hancock watching Luffy being affectionate with Anne (A Gender Flip Ace) with jealousy, even thinking that Luffy might prefer Anne's petit chest over Hancock's more voluptuous pair. She doesn't know that Anne is Luffy's adopted older sister.
- Ma Fille: In the chapter "Sweet Baby Sakura", Natsumi's doula, Nurse Mahari, mistakenly believes Piston Hondo to be her partner. Hondo, in a rare moment of immaturity, is grossed out and clarifies he's her brother. Mahari trolls him a bit by pointing out the age gap between him and Sakura, acting like she thinks Hondo is Sakura's brother and not Natsumi's.
- When Oscar sees Ruby and Yang hugging in RWBY Thoughts, he mistakes them for girlfriends instead of sisters.
- In Resilience of the Soul, Linca's Backstory involves the time she became upset when she saw her lover hugging a pregnant woman who turned out to actually be his sister. It's not funny though — before the matter can be cleared up, she accidentally kills him.
- In Comes and Goes (in Waves), everyone believes that Five and Raithe (Ghost Klaus) are together. Everyone. It doesn't help that they're Cuddle Bugs who have no trouble with casually saying a Platonic Declaration of Love whenever the mood strikes them, thanks to spending over two decades in the apocalypse with only each other as company. They're aware of and extremely Squicked by the misconception (especially since Raithe pretty much raised Five), but it's so pervasive they hardly even try to correct it anymore.
- The divergences in With This Ring include Superboy being adopted by Wonder Woman, who of course looks not much older than him (and people do look). Some of his school peers wonder if he's cheating on "Megan" (Miss Martian) when they see Diana hug him and kiss his cheek, until they find out what's actually going on.
- Young Justice: Darkness Falls: In chapter 6, Starfire explained the reason why she kissed Nightwing (to acquire his language) instead of Conner was that the latter was with his mate (M'Gann, whose relationship with Conner is... complicated at that point) and their offspring (Beast Boy, who is really M'Gann's adopted brother).
