Warning: Spoilers Ahead.
Relatives getting mistaken for Love Interests in Live-Action TV series.
- 45 RPM: Maribel mistakes Robert's sister Clara for his ex-girlfriend when she first sees them together.
- The Beauty Queen Of Jerusalem: Luna sees Itamar on a date with another girl, and walks up and starts insulting her, before Itamar informs her that's his sister.
- In Black Books, Fran gets annoyed when she sees her boyfriend having coffee with another girl, that turns out to be his sister. Partially lampshade hung when Bernard guesses, correctly, that that was the case before Fran actually reveals that to him. They break up anyway, but only over of how Fran jumped to conclusions and behaved childishly. Her boyfriend comments that he wants to date someone less "mad".
- Boardwalk Empire: New York gangster "Lucky" Luciano and Atlantic City gangster Jimmy Darmody quickly develop a mutual dislike of each other. So when Luciano meets a stunning redhead who introduces herself as "Mrs. Darmody", Luciano quickly makes the moves on her, assuming she's Jimmy's wife. When his boss Arnold Rothstein calls him up, he's quite amused, telling Luciano that he's sleeping with Jimmy's mother. Said mother later even helps out her boy with luring Luciano into a trap.
- Used secondhand in Bones, where Booth and Brennan saw Sweets' girlfriend in a wedding dress hugging another man, and after some disagreement Brennan told him, made worse by her directly informing him that his girlfriend was engaged to someone else. Luckily for them when he confronted her about it she explained what had actually been happening (she was trying on a wedding dress for her cousin, who's the same size), and they... reconciled.
- ''The Carol Burnett Show": In one skit, Carol and Vicky play friends having lunch in a nice restaurant, when Carol spots Vicky's husband (Harvey) and a younger woman seated at the table behind Vicky's back. Assuming he's cheating, Carol does everything in her power to distract Vicky from noticing, including tripping their waiter, played by Tim Conway, no less than three times as Harvey walks past their table. Harvey finally spots Carol and calls her & Vicky over to his table to introduce Carol to his "much younger sister," much to Carol's chagrin.
- Played with a lot in Castle, where Rick says to Beckett more than once things like 'I've got a big date' or that he has a lady to see when he's actually just talking about his daughter Alexis. As a Genre Savvy writer, he does this deliberately to feed the Unresolved Sexual Tension and make Beckett jealous, which she does a poor job of hiding for someone trying to convince everyone (including herself) that her relationship with Rick is strictly professional. It also shows up in a more conventional form in one episode where a famous baseball player's wife suspects him of an affair after he's seen with an attractive young(er) woman — the younger woman is actually his daughter from a previous relationship, who assures the woman (technically her stepmother... of sorts) that her father really did love her.
- Exploited and Played for Drama in the Cold Case episode "Thick As Thieves". As the detectives eventually find out, the Victim of the Week and her boyfriend were a team of con artists who claimed to be a couple as part of their swindling strategy, when the truth is that the "boyfriend" was the victim's son.
- Tracy Barlow invokes this in Coronation Street when her brother Peter is upstairs as her abusive boyfriend Charlie arrives home. Tracy pretends that she has brought a guy home but quickly shouts that it's her brother when Charlie rushes upstairs. Unfortunately, he finds Peter shirtless (he'd just had a shower) and beats him up.
- In Desperate Housewives, Lynette pretends to be a teenage girl in a chatroom so she can have real conversations again with her son. Unfortunately, he gets a crush on her online persona and she has to let him down easy with a goodbye message...which she signs "Love, Mom."
- In an episode of Drake & Josh, Josh sees his ex-girlfriend Mindy hanging out with and being affectionate with what he assumes is a new boyfriend. Of course, he turns out to be her cousin. When Josh asks why Mindy didn't say so in the first place, she says that Josh wasn't letting her explain.
- Firefly: When the box containing River is opened, Mal says scornfully, something to the effect of "Is she your lover, because you do seem awfully worried." Specifically, he accuses Simon of human trafficking, then sarcastically says something like "Oh, I'm sorry. Is this one for you, is it true love?"
- An episode of Frasier had Niles shopping for an engagement ring, accompanied by Frasier and Martin. Everyone in the store assumed the brothers were getting engaged to each other and applauded, much to their father's dismay. Seeing as how they do everything together, including going to their high school prom, this has probably happened to them a lot.
- Used in The Fresh Prince of Bel-Air, in which Hilary and Carlton were each flirting (separately) with an online admirer. They went to the restaurant to meet their date... only to discover that it was each other.
- Friends:
- Rachel is shocked that her boyfriend seems to be cheating on her, only to find that the woman he was hugging was his sister. Then Rachel goes to their apartment and finds them tickling each other, eating whipped cream from each other's laps and, ultimately, sharing a bath together. Her boyfriend insists that it's all innocent and just proof that they're close enough as siblings to behave as they did as kids, but Rachel thinks it's too weird and leaves him.
- Siblings Ross and Monica Geller, in spite of never having on-screen moments that may confuse people, are horrified to learn in the tenth season that they had actually kissed while teenagers. (It was dark, Ross thought he was kissing Rachel, Monica thought he was her "Midnight Mystery Kisser.")
- A brief bit in "The One Where They're Up All Night" where Rachel sees a picture of a girl on her assistant/boyfriend Tag's desk and says "Who's this chippy? Seems a little young for you, but whatever." She's his sister.
- Inverted in an episode where Joey, trying to get back together with an ex-girlfriend, arranges for a double date by telling his ex that Monica is his girlfriend while pretending to Monica that he's got a date with his ex who will be bringing her brother along as a date for Monica. Monica grows increasingly uneasy over the course of the evening at the behavior of the people she thinks are siblings. She figures it out when she sees the guilty look on Joey's face.
- Full House had an episode where Kimmy and Stephanie see DJ's boyfriend Steve with another woman, but the woman was later revealed to be his cousin.
- Gilmore Girls
- When Richard is in the hospital, Michel hijacks Sookie's concerned phone call to inform Lorelai that a "couple" who checked into the Dragonfly Inn is actually a brother-sister pair.
- In Season 6, Zack, who has recently broken up with Lane, is jealous when he sees her hanging around a Korean guy and confronts her mother about it. It turns out the guy is a young uncle.
- Subverted in Good Luck Charlie: Teddy sees Spencer with a girl named Skylar and confronts him about it, only for Spencer to inform her that Skylar's actually his cousin. He's lying — he's dating Skylar on the side and that Skylar, who had no idea that Spencer already had a girlfriend, was under the impression that Teddy was his cousin. When the two girls eventually realize the truth, neither one takes it well - -especially Teddy.
- In one episode of Brazilian TV show A Grande Família (The Big Family), Isabel "Bebel" Carrara assumed her husband was having an affair. Turns out he was just meeting his long-lost sister.
- Grey's Anatomy introduced Patrick Dempsey's sister this way.
- In the third season of High School Musical: The Musical: The Series, Jet and Maddox don't tell anyone they're siblings because Maddox is embarrassed by him and Jet doesn't want to be at summer camp. But this causes a rumour to briefly spread that Maddox has a crush on Jet, and that persists until a public fight causes everyone to realize the truth.
- How I Met Your Mother featured an extended flashback to Barney's first love. His girlfriend was supposed to go into the Peace Corps with him, but bailed at the last moment with the excuse "My father won't let me go." Barney goes to give the "father" a piece of his mind and walks in on them kissing.
- In Keeping Up Appearances, when Hyacinth sees Emmett, her neighbour Elizabeth's brother, for the first time, she believes her to be cheating on her absent husband. Although it isn't because she sees him with her, it's because she sees him at her house, dressed only in a towel.
- Malcolm in the Middle:
- Francis' wife Piama is meeting with a man from her past that she doesn't want to introduce him to. Francis' friend convinces him it must be her recently paroled ex-husband and they're probably hooking up, causing Francis to storm home and confront them. Turns out the man is Piama's estranged father looking for money, and she was just planning on chewing him out and turning him away while Francis was at work.
- Inverted in when Reese walks in on his wife cheating on him and she tries to pass off her lover as her brother. Having turned into an Extreme Doormat by then, Reese buys it, and even continues to call him her brother when he later walks in on the two of them in bed.
- In one episode of Mighty Morphin' Power Rangers set after the marriage of Lord Zedd and Rita Repulsa, Rita greets a man whom she actually seems happy to see. Zedd is (to his own surprise) rather perturbed by this until Rita introduces the new arrival as her brother, Rito Revolto.
- Murder, She Wrote
- In the episode "The Grand Old Lady", a married soldier coming home from World War II had a relationship with his nurse, who accompanied him back to the US because she couldn't bear to leave him until she had to, but intends to remain on the ship and go straight back to Britain once he's reunited with his wife. Once the murder's solved and everyone's finally able to disembark, she sees a woman running up to him and embracing him, and turns away. The next time the soldier and nurse appear, they're together, with the soldier cheerfully explaining that his sister met him at the docks to tell him his wife had run off with a lumberjack.
- In "A Christmas Secret", Beth knows her fiance Charlie has some kind of secret, and follows him to see him embracing a woman at the Lighthouse Motel. When Jessica introduces herself to the woman, she explains she's Charlie's sister, Monica.
- One episode of Power Rangers Lightspeed Rescue had Ms. Fairweather call a famous scientist named Clark to help her build some armour for the Rangers. They get along exceedingly well, and Clark is even given the honor of revealing her first name (Angela) to the audience. Meanwhile, Joel, who was in love with her, is seen getting very, very jealous, and even tries to sabotage one of their 'dates.' Finally, they get attacked by a Monster of the Week and, seeing Ms. Fairweather's obvious distress when Clark is still in danger, Joel decides he wants his beloved to be happy and saves his rival. Later, he gamely tells her how glad he is for her and that she deserves the best boyfriend ever. Naturally, this is when he is treated both to the reveal that he was her brother all along and to a kiss on the cheek. In a No Hugging, No Kissing series, even. Joel and Ms. Fairweather have the distinction of being the first PR Official Couple to get married.
- Remington Steele: Steele once saw Laura talking with her brother-in-law and thought she was having an affair. It didn't really help that the brother-in-law kept kissing her on the cheek...
- In Reno 911!, the sheriff department notice Dept. Garcia embracing a beautiful woman. They're all happy for him for finally finding somebody, but then later they see that same girl with another guy. They all decide to do Garcia a favor and run the guy out of town. The next day, a depressed Garcia comes into work and tells everyone how his sister's fiancé left without any warning and that she's very upset over it.
- In Sabrina the Teenage Witch, Hilda and Zelda go to Parent-Teacher Night, and Zelda is attracted to Mr. Pool. After introducing themselves as Sabrina's aunts, she quickly adds that they're sisters, "not an alternative couple."
- Schitt's Creek: In Season 1, Alexis thinks her crush Mutt is having an affair with the mayor's wife, Jocelyn. Turns out Mutt is Roland and Jocelyn's estranged son. Given that Jennifer Robertson is only five years older than Tim Rozon, you can see why she might be confused.
- Supergirl (2015):
- When Supergirl surfaces for the first time, people wonder whether she's somehow linked to Superman. Kara Danvers visibly cringes when she listens to her boss suggesting they may be lovers.
- Cat's emails get hacked and Kara, Jimmy, and Winn are tasked with going through them to find anything potentially scandalous. They find evidence that Cat has been wiring money monthly to some unknown twenty-four-year-old man. When Kara brings this up to Cat — certainly not suggesting that she might be paying for a male escort or something! — Cat explains that he's her estranged older son.
- Kara and Alex have been mistaken for a couple at least once. They're foster sisters.
- The Winchester brothers in Supernatural are frequently mistaken for a gay couple. This apparently does not stop being funny, ever. In this Crapsack World, you take laughs when they come.
- Taboo: Subverted when Lorna Bow sees James Delaney paying a lot of attention to Zilpha during an upper-class party they were invited to. The two women later run into each other in the ladies room, and Lorna inquires of Zilpha whether she should consider her a rival for James's affections. Zilpha defiantly states she has no cause to worry since she and James share the same father. The subversion results from the fact that the two half-siblings actually used to be lovers as teenagers and still share a large amount of sexual tension.
- In True Blood, Violet Mazurski assumed Jason Stackhouse was cheating on her when she saw him hanging out with Sookie until he told her Sookie is his sister. Violet's response is to say, "Then you're my sister too!" and then kiss her on the lips.
- In The Tudors, Henry spots Anne talking with another man; later, during an illicit meeting between them, wraps a hand around her throat and demands to know who he was. As Anne explains, and the audience already knows, the other man was her brother, George.
- Two and a Half Men features brothers Charlie and Alan acting a lot like a married gay couple, which is a running gag in several episodes. For instance, on one occasion Alan's out camping, Charlie's asked by a friend about "the wife and kid."
- In The Umbrella Academy, in Episode 8 the siblings are hiding out at a children's bowling alley. Luther and Allison head outside to deal with another issue, leaving Diego and Klaus to watch over Five (actually the oldest of the brothers but currently stuck in a young teenage body.) A friendly mother approaches them and:Mother: Excuse me, it's my son Kenny's birthday today, and wouldn't you be happier playing with kids your own age? Assuming it's OK with your dads.
- In the last season of Veronica Mars, Mac's newly introduced animal-rights-activist love interest turned out to have a female housemate, who triggered this trope.
- The Vicar of Dibley:
- Geraldine falls in love with Harry and gets jealous when she sees him walking around the village with another woman. It isn't until he shows up at the vicarage to propose that she finds out the woman is his sister. Even then, she initially mistakes his request as being for her to officiate the marriage, a Call-Back to a previous episode where the guy she thought she was dating had meant that.
- Variant: An earlier episode had Geraldine nearly call off Alice and Hugo's wedding when she thought that David Horton, Hugo's father, was also Alice's dad via a one-night stand. Turned out the David in question was a (sufficiently distant) relative of the show's David who had the same name.
