Double In-Law Marriage

Bob and Janet are siblings. So are Spencer and Alice. Alice marries Bob, while Janet falls in love with Spencer. This would also work if Alice and Janet were sisters, and Bob and Spencer were brothers. Then Alice would marry one brother, and Janet would marry the other. In other words, a pair of siblings marry another pair of siblings. Someone marries the sibling of their own sibling's spouse. A quick way to achieve Weddings for Everyone. Or if the weddings don't take place at roughly the same time, the first wedding provided the ideal opportunity for the second couple to get to know each other.

This can lead to a major Tangled Family Tree as the children of one couple are "double cousins" of the children of the other. Since they are as genetically similar as half-siblings, Kissing Cousins is even more discouraged in such cases. This gets ramped up in cases of one or two sets of identical twins being involved, for obvious reasons.

Not to be confused with Brother-Sister Incest or Settle for Sibling (although overlap with the latter is certainly not impossible).

Anime and Manga

 * In Wild Rock, Yuuen's brother and Emba's sister meet during their ceremony, which leads to the Babies Ever After chapters.
 * In Mahou Sensei Negima's finale, it's shown that the twins Fuka and Fumika ended up marrying twin princes from the magic world.

Comics

 * This is sometimes the origin for Peter Parker's Aunt May and Uncle Ben's relationship with his parents. Originally, Uncle Ben was just Richard Parker's considerably older brother and Aunt May and Mary Fitzpatrick-Parker were unrelated, but some recent versions have the women as sisters as well.
 * This is specifically hinted at in Ultimate Spider-Man, where Aunt May refers to her sister's death and calls Peter her sister's responsibility during a fight.
 * Explicitly done in Mark Millar's Trouble, one of several reasons it was immediately declared Canon Dis Continuity.
 * In the regular continuity, however, it's often noted that this is not the case, and Aunt May (Reilly, not Fitzpatrick) and Peter Parker are not related by blood at all, making her selfless love and doting nature all the sweeter.

Fan Fiction

 * In How I Became Yours, Zuko gets together with Katara... and Azula with Sokka.

Fairy Tales

 * The eponymous sisters of "Snow White and Rose Red" married princes who were brothers.
 * In King Arthur mythology, brothers Gareth and Gaheris married sisters Lyonesse and Lynet.

Literature

 * Jane Austen loves this trope:
 * Northanger Abbey: John and Isabella Thorpe plan to marry Catherine and James Moreland, respectively, but ultimately don't because John is a pompous windbag and Isabella is an incurable flirt.
 * Sense and Sensibility: John Dashwood and his half-sister Elinor Dashwood marry sister and brother Fanny and Edward Ferrars. John and Fanny are already married before the book begins, but Elinor and Edward don't get married until the end (Edward's Arranged Marriage and Childhood Marriage Promise to Lucy were in the way).
 * Mansfield Park: Mrs. Grant plans for her brother and sister Henry and Mary Crawford to marry siblings Julia and Tom Bertram. They both blow it, starting by flirting with the other two Bertrams Maria and Edmund. Note to Shippers On Deck: Make sure the sides are balanced before attempting this trope. Ironically, at least 2 Fan Sequels -- Susan Price and Mansfield Revisited -- invoke this trope by marrying Fanny's sister Susan Price to Edmund's brother Tom Bertram.
 * Emma: Brothers John and George Knightley marry sisters Isabella and Emma Woodhouse. As in Sense and Sensibility, the heroine's marriage takes place at the end, while her sibling's marriage took place before the start of the book, so Emma and her Mr. Knightley have been Like Brother and Sister all her life.
 * At the very beginning of A Song of Ice and Fire, best friends King Robert and Ned Stark intend to do this. They had already betrothed Sansa and Joffrey, while Robb and Arya escorted Myrcella and Tommen to dinner, with the implication that they were trying to set them up. Later Catelyn Stark makes a contract with Walder Frey for Robb and Arya to marry two Frey siblings.
 * A minor side-plot in Tom Clancy's Clear and Present Danger concerns a pair of criminal twin brothers, who fell for a pair of twin sister hookers, and got sent to prison for murdering their pimp... they wind up getting out again after a little unofficial deal is struck with the police, killing a pair of Cartel Goons who were set to get off on a technicality, inside the prison. As far as we know, they all lived Happily Ever After while the main plot moved on...
 * In Anne's House of Dreams, Leslie's husband Dick had a "double cousin" named George who looked almost like his twin.
 * In Amy Tan's The Bonesetter's Daughter, sisters Lu Ling and Gao Ling marry brothers Edwin and Edmund Young.
 * In The Bacta War, we learn that Lanal Darklighter is Gavin's aunt on both sides of the family, because she is both his mother's sister and his paternal uncle's wife.
 * There exists somewhere an "ending" written for the Arabian Nights that was made several hundred years later, in which Shahryar had magically fallen deeply in love with Scheherazade, decided to keep her, and then his brother magically fell in love with her sister so they could be one big happy family.
 * Mentioned in an aside in To Kill a Mockingbird, although not actually featured. Atticus mentions the concept of double cousins to Scout, saying "two brothers married two sisters." She and Dill work on figuring it out, and reason that if Dill had a sister whom he married, and Jem and Scout got married, their kids would be double cousins. Only off by a little bit there...
 * Used in Ada, or Ardor to set up the incestuous cousins, although they later turn out to be siblings as both were the product of an affair between the double in-laws.
 * In the novel Malevil, the main character's father and uncle married a pair of sisters.
 * In Emily Rodda's Pigs Might Fly, this happened to Enid and her sister, who met their future husbands (brothers) at the same party. This is justified as it happened during a UEF (Unlikely Events Factor) Storm, which causes unlikely events such as pigs flying.
 * This shows up several times in The Silmarillion and The Lord of the Rings.
 * Sam's sister Marigold Gamgee married Rose's brother Tolman Cotton.
 * Galdor, heir to the house of Hador, and his sister Gloredhel marry Hareth of the Haladin and her brother Haldir.
 * Galdor and Hareth's sons Húrin and Huor marry first cousins Morwen and Rían.
 * Siblings Éomer and Éowyn of Rohan marry first cousins Lothíriel of Dol Amroth and Faramir of Gondor.
 * Bilbo's father (Bungo Baggins) and his first cousin Rosa married siblings Belladonna and Hildigrim Took. (Bilbo's and Frodo's fathers were first cousins once removed, and their mothers were aunt and niece--look through the family trees at the end of The Lord of the Rings and you can find endless cases of people marrying their in-laws as well as Kissing Cousins.)
 * In Shanghai Girls by Lisa See, sisters Pearl and May marry brothers Sam and Vern Louie, and Pearl has a daughter.
 * In Jacqueline Wilson's Double Act, Garnet has always assumed she and her twin sister Ruby would marry a pair of twin boys.

Live Action TV
"David: Thanks for making me and my girlfriend related."
 * Roseanne: Darlene and Becky Conner married David and Mark Healy, respectively.


 * Or, according to the finale.
 * The characters had a lot of fun joking about David and Darlene's relationship after they essentially adopted him as a member of the family long before Becky got married. To the point where when Darlene and David get engaged Roseanne celebrates the fact that they are officially the most redneck white trash in town.
 * In Lost in Austen, after Mr. Collins marries Jane, he tries to marry his brothers off to the younger Bennett sisters. It doesn't quite work out.
 * Into the West used this in the first episode.
 * Howe & Howe Tech is about a company run by identical twin brothers Mike and Geoff, with their wives, Tammy and Tracy, handling office work. Tammy and Tracy are sisters.

Religion and Mythology

 * A rabbinic legend says that The Bible's Laban intended to marry his two daughters to their two male cousins--the older two, Leah and Esau, would be one couple and Jacob and Rachel would be the other. Averted because Leah prayed to God that she wouldn't have to marry the wicked Esau, and He arranged for both sisters to marry Jacob instead.
 * In Classical Mythology there were the Danaids, fifty sisters, who were in a mass Arranged Marriage to their fifty male cousins. They are eventually compelled to go through with the marriages, and then.

Tabletop Games

 * The Clothar race in Traveller always has these. Clothar are born as a brother sister pair of mindlinked twins and each pair marries another pair creating what amounts to a quadruple marriage rather then a dual marriage. As each pair is telepathic, effectively everyone knows everything about the others marriage.

Western Animation

 * A Disney Silly Symphony ended like this. Two feuding royal families make peace, and the king and the prince of one kingdom marry respectively the queen and princess of the other kingdom (by the way, they were also all anthropomorphic musical instruments).

Video Games

 * Doable in Fire Emblem: The Sacred Stones, pairing Ephraim with Tana and Eirika with Innes.
 * Also doable in the second generation of Genealogy of the Holy War with, well, any two brother-sister pairs. Could potentially be two pairs of Kissing Cousins if you pair Patty with Lester and Faval with Rana.
 * Note that due to the genetics that come with their mothers being identical twins, both of the above pairings also count as Brother-Sister Incest.
 * A sidequest in Final Fantasy XII revolves around preventing this situation. A group of seven Archadian nobles who are all brothers are trying to force a group of seven sisters who work as airship stewardesses to marry them.

Real Life

 * Anthropologists call (one type of) this Brother-Sister Exchange, such as the Mundugumor of New Guinea.
 * The Yanomami do this a lot: A man marries his sister's husband's sister. The children of one marriage then typically marry the children of the other. Happens a lot with moieties (two groups which must marry someone of the other group).
 * Romans, especially the rich upper classes, would often do this, with daughters as bargaining pieces to enforce alliances. Just look at the family of the Julio Claudian Emperors, for example. This tradition carried over to the medieval era, with most royal families engaging in this.
 * The most famous Siamese Twins (now referred to as Conjoined Twins), Chang and Eng Bunker, married sisters.
 * It doesn't come up in her own books, but Laura Ingalls Wilder's parents were the second of three marriages between their families: Uncle Henry was Ma's brother and his wife, Aunt Polly, was Pa's sister, while Aunt Eliza was Ma's sister and Uncle Peter was Pa's brother.
 * There was a question on Yahoo Answers once (quite possibly Trolling, or at least posing a thought experiment) about a pair of identical twins who'd married another set of identical twins and wanted to set up an Arranged Marriage between their respective children. Once you've gotten over the initial instinct to reach for the Brain Bleach and done a bit of basic maths, you realise the kids in question would be, genetically speaking, full siblings. Yeeah. There was a case in the UK of one set of identical twins marrying another, not that they planned it like that. One marriage apparently ended in divorce.
 * Averted by the Catholic Church. Such a marriage is forbidden under the affinity impediment.
 * Sigmund Freud's wife Martha Bernays was the sister of Ely Bernays, who married Sigmund's sister, Anna Freud (making public relations pioneer Edward Bernays Freud's 'double nephew').