Sister Ship

""if x=y then f(x)=f(y)""

- Every algebra textbook ever

Alice dates Bob. Bob is friends with Dave. Dave is single. So is Alice's friend Carol. The Rules of Fiction™ dictate that either Alice and Bob break up or Dave and Carol get together. There is no other way around it. Can go on to include larger cliques; so long as there's an even number, no one will be single or date outside their clique, since we can't have them date outside their clique.

May lead to OT4, OT5, or OT6, depending on the size of the clique. If Alice and Carol are sisters and Bob and Dave are brothers (or Alice/Dave and Bob/Carol are brother and sister), will lead to so-called "double cousins."

An inherent part of living in a moiety system, wherein all families belong to one of two groups and Alicetonians can only marry Bobsylvanians. A subversion occurs within cross-cousin marriages, where an Alicetonian man will marry a Bobsylvanian woman but a Bobsylvanian man will marry a Carolstani woman but a Carolstani man will marry an Alicetonian woman. (In some cultures, it reverses every generation.)

Anime & Manga

 * Neon Genesis Evangelion gives us Shinji/Asuka and Kaji/Misato, with the subtext of Shinji/Misato and Asuka/Kaji. Freud Was Right, after all.
 * Mobile Suit Gundam Seed has Kira/Lacus and Athrun/Cagalli, with Kira's sister ending up paired with his best friend and Kira ending up paired with his best friend's former fiancee. (It's not hard to find Athrun/Lacus, Athrun/Kira, and/or Kira/Cagalli subtext as well.)
 * Mobile Suit Gundam Seed Destiny, on the other hand, messes with the formula by breaking up Athrun and Cagalli while Kira and Lacus stay together.

Fanfic

 * Many a Mary Sue is based on this, inserting a sibling or other close platonic associate for an existing character as an in to pairing them up with the sibling/partner/friend of that character's canon love interest - for example, giving Han Solo a sister and pairing her with Luke.
 * Some pairing combinations in the Axis Powers Hetalia fandom are done with this in mind. The most prominent is America/England + France (England's Sitcom Arch Nemesis)/Canada (America's brother) or, alternatively, England/France + their respective former colonies America/Canada with the FACE family. It's also half of the reason for Denmark/Norway being a popular Ship Mate for Sweden/Finland in the Nordic unit even before they got any real canon interaction.

Literature

 * In the Star Wars Expanded Universe, the Skywalker family and Hapan royal family seem to cross over with some frequency: Teneniel/Luke, Tenel Ka/Jacen, Isolder/Leia, and Taryn/Ben/Trista are all at least hinted.
 * Harry/Ginny and Ron/Hermione of course.
 * This configuration of ships is called "One Big Happy Weasley Family". And just to make it all the bigger and happier, we had Bill Weasley and getting married and their daughter being involved with.

Film

 * When Harry Met Sally: Harry and Sally fix each other up with their respective best friends. The two end up hitting it off and get together before the titular couple. So this is actually both an inversion and played straight.

Live Action TV

 * Degrassi the Next Generation loves this:
 * Kinda complicated. Jimmy dated Ashley, so of course their sidekicks Spinner and Paige have to date. When Paige went all Starscream on Ashley, Spinner was also elevated to stardom. Spinner's friend Marco started dating Paige's brother Dylan, so even homosexuals aren't immune.
 * Craig and Sean. Craig and Sean. Close friends in season 2 and Degrassi's first popular nonsquick slash pairing, they're also something of romantic communists. Craig dates Manny in season 2, and Sean dates Emma. Ditto for season 6. In the meantime, Craig dates Ashley, but while Ashley hates Craig for dating Manny at the same time, Sean dates Ashley's best friend Ellie. And of course, Sean dated Ashley before either of them even knew Craig. (Sean does not react well when Jay dates Emma, though...) Despite (or perhaps because of) hate for Craig/Ellie, Sean/Manny is amazingly popular.
 * Sean is introduced to Jay and Alex. At this point, Jay is the Big Bad and the only one of Sean's new friends who really matters. Jay is dating Alex. Sean dates Alex's best friend Amy.
 * Aversion: No Jay/Hazel pairing, Palex or no Palex. Adam never dated Alli during all of Eclare.
 * Danny and Sav became Heterosexual Life Partners after Derek was given a Face Heel Turn. Danny's only two girlfriends during that time? Both were Anya's sidekicks.
 * Manny and Emma are best friends. Jay and Spinner are best friends. Jay and Manny spend several seasons as Shrodinger's fiances, finally ending up really seriously going to get married. End of Season 9 Emma and Spinner get married.

Theater

 * In The Ring of the Nibelung, Siegfried was, of course, the only one worth of Brünnhilde. But Gunther wanted Brünnhilde, and also needed Siegfried to marry his sister so he has support from the world mightiest warrior. So, let's use a Love Potion which also induces amnesia. And yes, harmonians are claiming that's what happened in Harry Potter.

Western Animation

 * This is the whole point of the Distaff Counterpart team on the Venture Brothers episode "Dr. Quymn, Medicine Woman".

Real Life

 * In many cultures, men develop a "brother-friend" relationship. Only one. One will often marry the other's sister-in-law. Sometimes they even share wives.