Warning: Spoilers Ahead.
Relatives getting mistaken for Love Interests in Anime and Manga.
- Implied in Attack on Titan: After the death of his prostitute mother, Levi is taken in by Kenny Ackerman, who simply states that he knew her in the past. As time goes on Levi finds evidence that he is also part of the Ackerman family, and while not stated explicitly, it's easy to read Levi as suspecting that Kenny might be his father. When Levi finally asks Kenny how he knew his mother, he laughs and explains that she was his sister, making him Levi's uncle (something which the audience already knew).
- In Baka and Test: Summon the Beasts, Akihisa attempts to keep his sister who has just recently moved home hidden from his friends for fear of her embarrassing him. When they start getting suspicious and go to his house they believe that he has a secret live-in girlfriend, much to Himeji's denial: "Akihisa-kun, this bra is way too big for you. You need a smaller cup size."
- Betrayal Knows My Name: Toko and Tsukumo Murasame seem to fit this, even going so far as to sleep in the same bed.
- Cardfight!! Vanguard: In Episode 5, when Emi yells at Aichi for not doing his homework with her, Kamui thinks thinks the two are a couple. They're actually siblings.
- Demon Slayer: Kimetsu no Yaiba: Zenitsu mistakes Nezuko for Tanjiro's lover instead of sister and attacks Tanjiro out of jealousy before he learns the truth.
- Used in Dragon Ball Z: Krillin mistakenly believes that Android 17 and 18 are a couple rather than Half-Identical Twins.
- A horrific one occurs in Elfen Lied, where Kaede mistakes Kouta's sister for his girlfriend. She promptly kills her and his father.
- Inverted in Even Though We're Adults. Akari initially mistakes Ayano's husband for her brother. She didn't know that Ayano was married.
- Invoked with Miki and her brother Kazuki in Fresh Pretty Cure!. Love, Inori, and even Miki's admirer Yuuki mistake Kazuki for her boyfriend on multiple occasions, and he plays along to keep other boys from hitting on her.
- In Haikyuu!!, Tanaka sees Kanoka with an older man and assumes he's her boyfriend, much to his embarrassment (he was in the midst of rejecting what he thought was a love confession from Kanoka). It's only after he runs away that the audience finds out the man is her cousin. Tanaka himself finds out the next day.
- In Himouto! Umaru-chan, when Umaru and Taihei go out in public, bystanders often get jealous of Taihei, thinking that the beautiful Umaru is his girlfriend instead of his sister.
- Shiba Tatsuya and Shiba Miyuki from The Irregular at Magic High School are a pair of rather affectionate siblings. So much so that even their innocuous conversation with each other creates an atmosphere that makes others think about incest. It is no wonder that Ichijou Masaki mistook them for lovers when he first met them. His startled realization that they are actually siblings is Played for Laughs, at the same time, in the future, we first learn about the incestuous feelings of Miyuki, and then that Tatsuya is ready to try to love her as a woman. In general, at the moment they are engaged after a series of events ...
- Kenichi: The Mightiest Disciple: When Renka shows up, she furiously attacks Kensei. Given that Kensei is a Dirty Old Man, Kenichi immediately assumes that Renka is a disgrunted ex-girlfriend. An annoyed and disgusted Kensei explains that Renka is his daughter who is attacking him for abandoning the family.
- Zig-zagged in K-On! High School when the other Pop Music Club members see Jun meet a guy. Three of them think he's her boyfriend, but Ui is unsure, having heard she has a brother, but they assume "boyfriend status confirmed" when she squeezes his arm. But then they learn after confronting them that he actually is her brother Atsushi.
- In Love Monster, Hiyo assumes that Kurou is flirting with Chiya, but in reality, he just found out Chiya is his sister. Later on in the story, a squicky situation occurs where Chiya does actually try to pursue a relationship with Kurou, which is perfectly acceptable in monster society. Kurou is not interested, however.
- Macross 7 has a variation when Mylene visits Max Jenius on the bridge to talk to him. She's never been on the bridge before, and after they leave to talk in private, the Bridge Bunnies go into a frenzy over Max's new "girlfriend" who is so much younger than he is, some of them scandalized and others grudgingly admiring Max for attracting someone so young and beautiful. Then Exsedol cuts in and informs them all that Mylene is Max's daughter. (Mylene's somewhat risque manner of dress probably wasn't helping matters.)
- In My Love Story!!, a man named Oda confesses to Ai and gets turned down because Ai is still in love with Takeo. He goes to meet Takeo however mistakes Suna, Ai's Bishōnen younger brother, for Takeo. Ai described Takeo as very attractive looking however Takeo isn't what Oda expected.
- No Matter How I Look at It, It's You Guys' Fault I'm Not Popular! has this happen a lot with Tomoko and Tomoki. The two don't have the most loving relationship and as a result tend to act snarky towards one another, but when other students see them interacting, they mistake the behavior as a sign of closeness or flirting. Which creates complications because a number of girls who make this mistake, which so far include Akari and Sachi, are crushing furiously on Tomoki and subsequently start viewing Tomoko as a rival for his affections. Hilarity Ensues.
- It is possible to see this trope at work in Rebuild of Evangelion, counting Rei's affection for Shinji as purely maternal — if this is the case, Asuka makes a fateful error.
- Sailor Moon:
- Used in the English dub to Hide Your Lesbians. Serena and Mina mistake Amara for a man and assume "he" is dating Michelle when they are actually cousins. Of course in the original Japanese they were a couple and the dub didn't edit out that many couple-y scenes between them.
- Another variation in the R season. Serena/Usagi spots Darien/Mamoru giving another girl a lift and assumes he's found another girlfriend. The girl turns out not to be his sister but rather Andrew/Motoki's sister though the set-up remains the same. The sister has a boyfriend already anyway.
- School Rumble plays with this one: After Harima sees Tenma and Karasuma together, he sulks in his room and ends up watching "Comet Wars", which mirrors the Star Wars use of this trope. Harima suddenly realizes he may have made this mistake and that Karasuma is Tenma's brother... but later realizes he was just fooling himself.
- Invoked in A Silent Voice: Shouya meets a boy who says he's Shouko's boyfriend. The awkward part is that Shouko is around seventeen while the kid is a middle schooler. As in turns out the two are related, with Yuzuru being Shouko's overprotective little sister.
- In one episode of Tokyo Magnitude 8.0, a man mistakes Mari for Yuuki's and Mirai's mother. She doesn't react negatively and when Mirai complains about it later she just mentions her own young daughter.
- Variation in Tonari no Kashiwagi-san. Yuuta sees his crush Kotone with Kotori's husband Takuya and initially believes that there's cheating afoot, unaware of the fact that Kotone and Kotori are sisters. The misunderstanding gets cleared up almost immediately.
- True Tears, Episode 3: Shinichiro sees Noe ride off on a motorcycle with another guy, who he assumes is her boyfriend. He finds out otherwise when he takes her to a basketball game and sees the same guy...Shinichiro: (to his guy friend) That's him! That number four. He's Noe's...
Noe: Brother!
Shinichiro: ...brother. (pause) Brother?! - In Weiß Kreuz, Omi and Sakura both assume that Aya-chan is their Aya's lover when they find out he's been taking care of a girl in a coma and using her name. The Assassin and White Shaman manga has a scene where Aya-chan forces her brother to leave his job early and buy her something to eat, and his coworker, implying that this has happened before, assumes she's his girlfriend.
