Warning: Spoilers Ahead.
Relatives getting mistaken for Love Interests in Western Animation.
- The Amazing World of Gumball:
- In the episode "The Flower" Gumball is jealous of Leslie because the latter has been hanging out with Penny more, but it turns out that Leslie is Penny's cousin—it should also be noted that Leslie is Ambiguously Gay.
- "The Ad" has the Wattersons hatch a plan to make extra money by renting their house to an elderly goat couple, Bernie and Ethel, while still secretly living in the house. When Bernie and Ethel are accidentally sent into a fainting spell (turns out they're fainting goats), the family stages photos of them at various vacation spots in a Weekend At Bernies-esque montage. The plan almost works... until Bernie and Ethel notice a photo of "them" on a romantic date at the burger shop. Turns out they're brother and sister. Oops.
- Arthur: In "Mr. Ratburn & the Special Someone", the kids learn that Mr. Ratburn's engaged to get married. When they run into him in a local diner with a woman named Patty, they assume that she's soon-to-be wife, and based on her abrasive personality, they try to sabotage the wedding for his own good. Not until the wedding itself do they learn that she is in fact his older sister, and the officiant — it also turns out that Mr. Ratburn's marrying a mannote and not a woman.
- In the Bob's Burgers episode "Boywatch" Tina becomes Shipper on Deck to two of her fellow trainee lifeguards.Tina: So, what do you think of Jason? I'm pretty sure he's into you.
Willow: Jason's my brother.
Tina: Oh. I was wondering why you both said "Bye Mom" to the same lady. Still, there's something there. - In the Camp Lazlo episode "Spacemates", Chip and Skip manage to do this to themselves.Chip: Are you my soulmate?
Skip: No, I'm your brother.
Chip: ...Oh. This universe is so confusing. - Captain Planet and the Planeteers: In the episode "Missing Linka", a disguised Wheeler helps saves Linka's life in her hometown in Russia. He initially assumes that a guy named Mishka is her boyfriend, but in the end, Linka reveals that Mishka's actually her brother.
- In Codename: Kids Next Door, Nigel's girlfriend, Lizzie, discovers him with another girl (an operative who goes by "Numbuh 10"). Thinking they're on a date she confronts him about it creating a huge scene. Nigel and Numbuh 10 explain that they had actually recently discovered that Nigel's mom and Numbuh 10's dad are brother-and-sister, which makes them first cousins. Instead of being relieved, Lizzie gets disgusted, still believing that it's a date.
- Inverted on Daria — when Daria first starts dating Tom, Kevin and Brittany initially assume that he's her brother. It seems to be partially because they think it's weird for someone like Daria to date, and partially because they know Tom was recently involved with Daria's friend, Jane.
- A newscast in DC Super Hero Girls calls Supergirl the "kid sister" of Superman. She's actually his cousin (and his older one, at that).
- Family Guy:
- Brian is trying to build a friendship with Quagmire, who he recently discovered actually hates him. At one point he sees a disheveled woman in Quagmire's doorway and, knowing the kind of guy that Quagmire is, starts making jokes about them having apparently just had sex. Quagmire angrily informs him that the woman is actually his sister who came over to escape her abusive boyfriend.Quagmire: I also have a deaf brother! Want to make fun of him, too?!
- He makes a similar mistake in "The Thin White Line" while making awkward small talk with his psychiatrist:Brian: I, uh, I notice you got a new receptionist. Nice little body on her, huh?
Dr. Kaplan: That's my daughter.
Brian: ... We could probably call this an early day, huh?
- Brian is trying to build a friendship with Quagmire, who he recently discovered actually hates him. At one point he sees a disheveled woman in Quagmire's doorway and, knowing the kind of guy that Quagmire is, starts making jokes about them having apparently just had sex. Quagmire angrily informs him that the woman is actually his sister who came over to escape her abusive boyfriend.
- In Garfield and Friends, "Wedding Bell Blues", Jon's cousin, Marian, in town for her upcoming wedding, comes over to visit her cousin, whom she hasn't seen in years. Marion explains to Jon that her soon-to-be husband's name is "Harold John," and when she points out the coincidence, she'll be "Marian John" (say it out loud). Garfield and Odie are standing outside the window of Jon's house when they hear Marian say this, but (not realizing that she and Jon are cousins) misinterpret their exchange as Marian being Jon's fiancée. Garfield soon gets worried that Jon won't pay as much attention to him after he gets married, as well as worrying that Jon's potential future children will play harmful games with him. In the end, he discovers what the viewer knew from the beginning: Marian is Jon's cousin and that she's marrying a man whose last name happens to be "John." His attempt to disrupt the wedding is fortunately taken in good humor (Harold in fact thinks the whole thing is hilarious) and the wedding continues.
- Jem: Aja and the others make this error in "Britrock". Aja's boyfriend starts acting more quiet than usual and tells her he has to tell her a secret he's been keeping. When Craig tells Aja Stormer is his secret Aja gets mad and it only gets worse when he introduces himself as "Craig Phillips" and Stormer as "Mary Phillips". Even Craig's bandmates lampshade how badly Craig went about the situation and decide to leave the room. Aja slaps Craig and runs away crying. It isn't until a moment later that Kimber informs her Stormer is Craig's little sister, not his wife. Craig had been hiding the fact because The Misfits and The Holograms are bitter enemies. Apparently, the fact Craig and Stormer are the only characters with deep blue hair went unnoticed by Aja.
- Les Sisters: Marine sees Maxence with a girl at the park and assumes that he's cheating on Wendy. Near the end, Marine and Wendy find out what the audience knew from the start, that she's only his cousin.
- My Little Pony Tales: In the episode "Sister of the Bride," Clover's sister, Meadowlark, is getting married to Cheval, and at one point in the episode, Clover and her friends see Cheval seemingly flirting with another girl, and they immediately accuse him of cheating on Clover's sister. Eventually, Cheval explains that the girl they saw him with was actually his sister.
- Next Avengers: Heroes of Tomorrow: of the main characters, four were raised together by Iron Man, while Francisnote was not, leading to this exchange:
- In the Phineas and Ferb episode "De Plane! De Plane!", Candace becomes jealous when she finds her crush, Jeremy, seemingly flirting with another girl, who later turns out to be his cousin — Jeremy explains that the conversation Candace overheard between was them was Nicolette (Jeremy's cousin) telling him about this video game level where she was stuck on, and how it took her at least a week to get out of it and move on to the next level.
- Regular Show: Mordecai sees a man bending down in front of his crush, Margaret, and overhears her saying "Yes dude, yes!" Thinking the man was her fiancé, Mordecai spends the episode in a depression. In the end, it turns out that the guy (named John) is actually Margaret's cousin, and that the exchange Mordecai witnessed was actually just John showing off his new prosthetic leg.
- In the last episode of Road Rovers, Hunter tracked down and went to visit a female dog from his past. Everybody, particularly Colleen, thought she was an old girlfriend. Turns out she was his mother.
- Rocko's Modern Life. In "S.W.A.K." Rocko is smitten with his new mail-carrier, Alicia, of which Heffer remarks Wallace from Pizza Face Pizza wouldn't be happy about. Rocko then spends the rest of the episode trying to keep Heffer — who had taken over Alicia's route while she took a sick leave - from delivering a love letter he wrote to her, in case it falls into Wallace's hands... which it does. Later, though, Rocko receives a response from Alicia, who notes that Wallace is her brother, much to his relief.
- The Simpsons:
- Lisa has a crush on an older boy, then hears him having a phone conversation with a girl, saying that he can't wait for her to get there. When the girl arrives, Lisa purposefully gets her lost in the woods, only to find out that her "rival" is, of course, the boy's sister. The boy, naturally, does not take this well.Lisa: Sister! You mean she's not your girlfriend?
Luke: Hell, no. They outlawed that in this state two years ago. - There is a variation in the episode "Bart vs. Lisa vs. The Third Grade"; some guys make fun of Bart by saying Lisa is his girlfriend after they see them holding hands. To be fair, the guys probably thought that Bart and Lisa were just unrelated friends/classmates instead of siblings.[Bart and Lisa are holding hands]
Guy #1: Hey, dude, who's your girlfriend?
Guy #2: Did you see his face?
[Bart and Lisa aren't holding hands]
Guy #1: Well, what happened, dude? Did your girlfriend dump you?
- Lisa has a crush on an older boy, then hears him having a phone conversation with a girl, saying that he can't wait for her to get there. When the girl arrives, Lisa purposefully gets her lost in the woods, only to find out that her "rival" is, of course, the boy's sister. The boy, naturally, does not take this well.
- Steven Universe: The Crystal Gems are Steven's guardians and Parental Substitutes, but Mayor Dewey assumed they were his sisters, an idea Steven finds too baffling to correct. Steven's father's cousin assumed Garnet was his biological mother. In a subversion, Sadie avoids assuming a specific relation, and simply calls them "Steven's... family?"
- In Voltron: Legendary Defender, Pidge has a picture that the others assume to be of him and a girlfriend. It's actually a picture of Pidge and her brother.
- In an episode of The Weekenders, Tish requests that she just hang out by herself for the upcoming weekend... and the others spy on her, suspecting ulterior motives, and see her hanging out with a boy her age all over town. When they confront her about her 'new friend,' she reveals first of all that it's her cousin Doug, and secondly, she had planned on being by herself that weekend, but Doug's visit was last-minute and her parents asked her to show him around.
- In an episode of X-Men: Evolution, Mystique brings Wanda into the Brotherhood house and Pietro has a huge Oh, Crap! moment when he sees her. During her rampage around the house, Toad asks if she's an ex-girlfriend. Pietro responds, "Worse! She's my sister."
