Warning: Spoilers Ahead.
Relatives getting mistaken for Love Interests in Live-Action Films.
- Inverted in After the Dark where it's mentioned that Jack once brought his boyfriend to a party and people thought they were cousins.
- In Archie: To Riverdale and Back Again, Jughead's son gets mistaken for Archie's son by his teachers at their high school reunion.
- Buffaloed: Graham sees Peg and JJ roughhousing at the bar and calls the cops, assuming it to be a "spousal altercation". They have to clarify that they are sister and brother.
- In Bugsy, Bugsy Siegel threatens to beat up a man with his pants off in his girlfriend's house. She's mending a hole in her brother's pants pocket.
- In The Dilemma, Ronny mistakes a man talking to his girlfriend as trying to feel her up. But when he confronts them, Ronny's girlfriend says that the man is actually her cousin — and he's also gay.
- Doom: The team of marines has a John Grimm. They meet a scientist named Sam Grimm and she and John clearly know each other. John's teammates assume she is his ex-wife until he explains she is his sister.
- In the 2009 film Dorian Gray, when Dorian introduces himself to Sybil Vane for the first time, he says he saw her previously in a gin tavern but was unable to talk to her because she'd left with another man. He's noticeably uncomfortable, assuming the man was her lover until she says that the man she left with was "Jim... my brother."
- G Men: When Brick, who has become attracted to Kay, sees her kissing Jeff (his boss), he's unhappy at first, but when he learns Kay is Jeff's sister, he smiles.
- The plot catalyst in Gray Matters. The brother-and-sister team both kiss the same woman, too.
- In Hobbs & Shaw, Deckard Shaw catches Luke Hobbs fighting Shaw's sister Hattie and furiously demands he get his hands off her, causing Hobbs to think she's his girlfriend. This naturally squicks both of them.
- The Holiday: Amanda accidentally sees the names "Sophie" and "Olivia" on Graham's cellphone and assumes they're two other women he's also dating. When she goes over to his house one night she learns that Sophie and Olivia are actually his daughters by his late wife.
- Kull the Conqueror: Kull assumes Ascalante to be Zareta's lover since he's so keen on rescuing her. When he asks Zareta directly about their relation to each other, she reveals that Ascalante is her brother. She later becomes Kull's lover, and Ascalante is totally fine with that.
- In Letters to Juliet, this causes Sophie some grief when Charlie introduces her to a woman named Patricia, who Sophie initially assumes is the ex-girlfriend he had mentioned earlier in the film. It turns out that the Patricia who Charlie was introducing Sophie to was actually his cousin, who just happens to have the same first name as the ex-girlfriend he had previously mentioned.
- Inverted in the Winona Ryder film adaptation of Little Women. A suitor of Jo's calls at the house, where Hannah the housekeeper mistakes him for a European friend of Amy's and informs him that "Miss March and Mr. Laurie are livin' next door". Friedrich takes this to mean that Jo has married her old friend Laurie, but Jo hurries down the road after him to explain that it's her sister who is married.
- Mabel's Blunder: Mabel sees her boyfriend embracing a woman and gets the wrong idea. It was his sister, and it was an innocent hug. But slapstick hijinx ensue nonetheless.
- In Mystic Pizza, Daisy is outside a country club with Kat and Jojo when she sees Charlie, her boyfriend, inside with a beautiful woman. In a rage, Daisy pours two barrels of fish into his car. Then Charlie introduces the woman (named Serena) as his sister.Daisy: [crying] I fucked up.
Charlie: Yeah, but you gave it 100% effort. - In Parasite, the Parks' daughter Da-hye notices that her English tutor Ki-woo is more familiar with her little brother's art teacher Ki-jeong than he should be. He'd told the Parks that she was a friend of his cousin's. She thinks they're dating but they're actually siblings.
- The French movie Qu'est-ce qu'on a fait au Bon Dieu uses this early on. An upper-class, white Catholic family has four daughters, three of whom have married an Algerian Muslim, a Sephardic Jew, and a Chinese Taoist. The final daughter is in a relationship with a Catholic... who's black. The three brothers-in-law take pictures of the fiancé with an attractive woman outside a hotel while in Paris, blissfully unaware of the hypocrisy of assigning stereotypes. When they show the sister the pictures, she asks what the hell is wrong with them, that's his sister. They genuinely apologize to him and do what they can to reconcile the families (as the fiancé's father is as opposed to his son marrying a white girl as her father to his daughter marrying a black guy).
- Slaughterhouse Rulez: Clemsie spends a lot of time hanging around Smudgers, touches his arm, and mentions that she loves him. The final act reveals that the two are siblings, not lovers.
- Star Wars VI: Return of the Jedi: Han briefly fears that Leia's avowed love for Luke puts him out of the picture, and promises not to "get in the way." But having discovered Luke is her brother, Leia is able to reassure him. (Of course, considering how vigorously she kissed Luke in the preceding film, Han's doubts weren't exactly unreasonable.)
- Swamp Thing: When Cable meets doctors Linda and Alec Holland, she's impressed by Alec's charm and brilliance, but when he takes her out on a tour of the swamp and starts hitting on her shamelessly, she angrily spurns him, saying, "Save it for your wife!" Upon their return to the lab, Linda makes an offhand reference to hers and Alec's father, revealing that they're siblings. Cable immediately starts treating Alec more warmly.
- Tango & Cash: Cash initially assumes Tango's sister to be his girlfriend. For that matter, so did probably most viewers, since it's never spelled out until Tango accidentally walks in on them. Which, when you think about it, unwittingly makes Cash far more of an asshole when he starts flirting with her behind Tango's back than he would otherwise be.
- Used in The Thomas Crown Affair, where the young hot girl he's seen dancing with and was in his bedroom is actually his adopted daughter. He could've easily told Catherine this, but he triggered this trope because he wanted to test her.
- A major plot point in Thoroughly Modern Millie. Millie sees Jimmy sneaking off at night with Miss Dorothy and assumes the worst. The movie ends with her realizing that she loves Jimmy...only to see him going off at night with Miss Dorothy and Muzzy. When she coolly confronts him and asks if she can join them, Jimmy cheerfully invites her to join him, his sister, and his stepmother. It turns out the three pretended they were unrelated because Muzzy wanted her stepchildren to find people who loved them for themselves.
- Early on in With Six You Get Eggroll, Abby McClure and Jake Iverson, both widowed with children, make a date for Friday night. Jake calls to cancel, citing his daughter's birthday, so Abby goes to a disco with her sister and brother-in-law, where they see Jake dancing with a pretty young girl. Abby gives Jake the cold shoulder but later learns that the pretty young girl is actually Jake's daughter when she sees her at her son's high school graduation.
- In Zathura, when Lisa finds out the astronaut she is attracted to throughout the movie is actually her little brother several years later, she cries, "Oh, my God! And I wanted to--" before an explosion cuts her sentence off. Needless to say, she was squicked and befuddled by it all.
