Spouses who adopt false identities pretend to be brother and sister, or vice versa
Characters who adopt false identities for prolonged periods, whether to avoid their enemies or to trick potential victims, sometimes change their apparent relationships along with their names. If a married pair of con artists or fugitives need to conceal who they really are, it's common in fiction for them to claim to be brother and sister, rather than husband and wife. The reverse is also seen a lot, when brother-sister duos pretend to be a married couple to conceal their true identities. This kind of pretense is a pragmatic option, as pursuers searching for a man-and-woman pair are unlikely to suspect a "married" couple of being brother and sister, or vice versa. Spouses who play at being single siblings can freely use charm, seduction, or even fraudulent marriage proposals as part of a scheme; siblings who claim to be married, conversely, can preemptively deflect any romantic entanglements that might otherwise endanger their facade. Resorting to this trope even makes the deception, itself, easier to execute: shared habits and (non-sexual) displays of affection don't necessarily have to be hidden, if spouses play-act at being siblings rather than unrelated friends or strangers. If spouses look less similar than is expected of a brother and sister, that dissimilarity may provide the basis for suspicion. Then again, it may be overlooked, lampshaded or handwaved. Variants in which couples pose as parent and child, or vice versa, also occur when age differences (or the appearance of such) are sufficient to make it believable. Related to Undercover as Lovers, either inverted or played straight. Lovers posing as siblings is a subtrope of Relatively Flimsy Excuse. Not to be confused with Brother–Sister Incest, in which siblings actually are a couple. As this trope involves characters with false identities, unmarked spoilers may follow. Examples: Anime & Manga
- The villains in the second part of Sailor Moon posed as brother and sister at school, but were actually lovers.
- Wild Wild West: Rita Escobar claims to be searching for her kidnapped father Professor Escobar, and when they meet the age difference makes this believable. However, at the end of the film Rita reveals that Professor Escobar is actually her husband. She pretended she wasn't married so she could use her sex appeal to get Jim West and Artemus Gordon to help her.
- In Tommy Boy, Tommy's dad marries a gold digger who is already married to the younger man she claims is her son.
- In The Hound of the Baskervilles, the Stapletons pose as brother and sister, but are actually a married couple. Jack Stapleton used this ruse to charm a lonely woman into being his accomplice, and to lure Sir Henry onto the moors in hope of seeing his "sister" Beryl.
- In one of Simon R. Green's Hawk & Fisher novels, the Battle Couple cops infiltrate a high-society gathering by posing as brother and sister. They speculate that they were assigned the role of siblings so they could sweet-talk information out of people; at one point, the fact their hair color is very different is inquired about, but Hawk deflects the question by claiming their mother was frightened by a seagull.
- In The Phoenix Guards, Phoenix siblings Illista and Allistar travel to Dragaera City by posing as a married couple of Issola.
- Genesis 12 and 20 give two accounts of Abraham disguising his relationship with his wife, Sarah, saying "She is my sister." Once this is to deceive the Pharaoh, and another time, to deceive Abimelech. In both accounts, Abraham does this so that they won't kill him to have his wife. Also in both accounts, the deceived takes Sarah, is warned by God that she is Abraham's wife, and returns her to Abraham.
- Genesis 26 describes Isaac and Rebecca having a similar incident with Abimelech. In this account, Abimelech finds out the truth when he sees the two flirting.
- Referenced in The Moving Finger by Agatha Christie: the protagonist, a convalescent pilot living with his sister (who looks very different from him) receives an anonymous letter accusing them of being lovers.
- In The Dragon Never Sleeps by Glen Cook: Lupo Provik appears in public with his female clone Lupo Two as "Nickla Ogdehvan" and his "wife." Everyone knows they're siblings, but that scandalous detail just distracts from the pair's actual identity.
- Robert A. Heinlein's The Puppet Masters. After agents Sam and Mary get married they go on a mission to check on the Pass Christian saucer, posing as brother and sister.
- A pair of con artists who were scamming both Charlie and his mother on Two and a Half Men posed as father and daughter (an older gentleman, and a nubile woman in Charlie's preferred age range), when they were really lovers.
- Doctor Who: In The Vampires of Venice Rory awkwardly introduces Amy as his sister, when it was technically the day before their wedding, to the Saturnyne Queen.
- This happened on an episode of The Andy Griffith Show, when a couple pretend to be father and daughter in order to run a scam on Barney.
- One episode of CSI: Miami had a married pair of murderers pretending to be brother and sister so they could run honeypot traps. One of their victims walked in on them and inferred something else entirely from the situation.
- Chibi Miku-san played this for laughs when Rin tricks Neru into thinking that she and her brother Len were lovers. A few strips later, Neru finds out she's been had by Rin the resident Troll.
- Sym-bionic Titan: the main trio, Lance, Ilana, and Octus, pose as a family when taking refuge on Earth. Octus takes double duty as brother and father as he is a robot with the ability to change form.
- In Real Life, ex-spouses Jack and Meg White — of the band, The White Stripes — often claimed to be brother and sister.
- Fantasy author Tracy Hickman has sometimes played along with the misconceptions of fans who assume he's co-author Laura Hickman's sister rather than her husband.
- Gay couples have sometimes claimed to be siblings to get around hospital prohibitions on visits by non-relatives. Couples who are legally married in a state or country with same-sex marriage may still resort to this pretense, if one is hospitalized in a jurisdiction where their marriage isn't recognized as valid.
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