- Marry your friend's sibling, so that your friend becomes your sibling-in-law.
- Marry your friend's child, so that your friend becomes your parent-in-law.
- Have a child who marries your friend's child, so that you and your friend become...um...well, in-laws of some sort, anyway.
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Anime And Manga
- In Code Geass, Lelouch Lamperouge considers this a favourable outcome with his best friend/rival Suzaku Kururugi and little sister Nunnally. They might have hooked up in the end, since Suzaku (as Zero) is seen working closely with Empress Nunnally, but Lelouch (aka the original Zero) isn't around to be an in-law.
- Implied by the ending of Fullmetal Alchemist. Brothers Edward and Alphonse have grown up with their neighbor Winry, and the three are extremely close. The story ends with Edward having married Winry, making her and Alphonse this trope.
- A few Shazam continuities have romance between Mary and Freddy, which would result in this trope for Freddy and Billy.
- In the Death Note AU Those Who Stand for Nothing Fall for Anything Matsuda is Light's brother-in-law through his marriage with Sayu, and he thinks he and Light are this. Light, for his part, finds Matsuda to be annoying but convenient, and uses him when he needs to look sociable.
- In the Harry Potter AU 3 Slytherin Marauders Snape and Lucius become this when Snape adopts Harry and Lucius marries Petunia and adopts Dudley.
- Any My Little Pony: Friendship Is Magic story where Twilight marries / gets together with Applejack's brother, Big Macintosh, as many fans of the series consider Twilight and Applejack to be closest to each other among the Mane Six.
- Happens, whether they like it or not, to Edward Elric and Roy Mustang in the Elemental Chess Trilogy. One of the stories in Notes From the Grandmaster confirms that Ed's son marries Roy's daughter.
- In any Sailor Moon fanfic where Hotaru marries Usagi's brother Shingo (a very popular pairing), she becomes Chibiusa's aunt as well as Usagi's sister-in-law. And since the other Outer Senshi are her adoptive parents, such a union would make more than half the team family.
Film - Live Action
- In Notting Hill, William Thacker's sister, Bunny, clearly believes in this trope when she first meets his date, famous actress Anna Scott:
Honey: Oh God this is one of those key moments in life, when it's possible you can be really, genuinely cool - and I'm going to fail a hundred percent. I absolutely and totally and utterly adore you and I think you're the most beautiful woman in the world and more importantly I genuinely believe and have believed for some time now that we can be best friends. What do you think?Anna: Ahm...I think that sounds, you know, lucky me. Happy birthday.Honey: Oh my God. You gave me a present. We're best friends already. Marry Will - he's a really nice guy and then we can be sisters.Anna: I'll think about it.
- The Town: Doug has an on-off relationship with his best friend Jem's sister, Krista. Jem's actually really enthusiastic about their relationship, as it means that he and Doug are basically family. However, it's a pretty rocky one and Doug dashes Jem's hopes that there's any future in it.
- The men in A Brother's Price hope for this and/or deliberately try to bring it about, as they can expect to see their family of origin much more often if the families are friends. For Ren, however, the marriage to her best friend's brother didn't work out so well; although Ren was good friends with Kij Porter, it turned out Keifer Porter's death was not a source of grief for any of his wives.
- At the end of the Harry Potter series, Harry becomes brother-in-law to both of his best friends: Ron and Hermione get married and Harry marries Ron's sister, Ginny.
- In the appendices to The Lord of the Rings, Sam's daughter Goldilocks marries Pippin's son Faramir.
- In the Star Wars Expanded Universe, this trope becomes canon for Luke and Han Solo.
- No one comes out and says it, but at the end of the Belgariad Durnik, Garion's oldest friend, technically becomes his uncle by marrying Polgara.
- Common in A Song of Ice and Fire, since the setting's tradition of Arranged Marriage is explicitly intended (at least in theory) to promote stronger ties between already-friendly nobles.
- The backstory has Ned Stark and his friend and mentor, Jon Arryn, marrying the Tully sisters, making them brothers-in-law.
- Robert Baratheon was supposed to marry childhood best friend Ned's sister Lyanna, which would have made them this. That fell through when Rhaegar Targaryen kidnapped her, leading to Robert's Rebellion and her death during it.
- Robert tries to invoke this trope by arranging marriage between his sons Joffrey and Tommen and Ned's daughters Sansa and Arya, explicitly comparing it to the betrothal between himself and Lyanna. Of course, there's also a political motivation; he wants Ned to accept an appointment as King's Hand, and this is a way of sweetening the deal.
- Ser Loras Tyrell and Lord Renley Baratheon aren't "just" best friends, of course, but the trope still applies: Ser Loras brokers a marriage between his sister Margaery and Lord Renley, among other reasons, so that Loras will have a good reason to spend a lot of time with Renley.
- At the end of Pride and Prejudice, Elizabeth marries Darcy and her sister Jane marries his best friend Bingley.
- In the Little Women series, Jo names her son after her sister's husband.
- In The Big Wave, Kino and Jiya are childhood friends, and Jiya grows up to marry Kino's sister, Setsu.
- Emma and Mr. Knightley start this way, because her sister is married to his brother, and for all their little quibbles with each other they are clearly devoted friends. They subvert the trope by getting married themselves at the end of the book.
- She also has something like this with Frank Churchill, whose father is married to her former governess and mother substitute, making them unofficial stepsiblings as well as good friends.
- In Íverenskommelser by Simona Ahrnstedt, Beatrice/Seth and Sofia/Johan have this kind of relationship. Beatrice and Sofia are cousins, while Seth and Johan were close friends even before they met the Löwenström girls.
- Magdalena and Gabriel in "De skandalösa" have a similar relationship with Gabriel's sister Nora and her gay lover Venus.
- Lord Peter Wimsey's Friend on the Force, Parker, falls in love with Wimsey's sister Lady Mary during Clouds of Witness and later marries her.
- In the "Billabong" series Jim and Wally are best friends and then become brothers-in-law when Jim's younger sister and Wally fall in love and get married.
Live Action TV
- Game of Thrones:
- Considering that Ser Loras Tyrell had dreamed of marrying Lord Renly Baratheon from a very young age (Renly is the "bride" in "green and gold brocade" - Renly wore a green brocade cloak to show his support for Loras at the jousting match), becoming brothers-in-law was the only viable way for Renly to become part of Loras' family (and not Loras becoming part of Renly's family - there is a distinction). Since he was a young teen, Loras had hoped to make this fantasy come true in some form, and his family's lofty ambitions would later become the perfect excuse to permanently bond Renly to the Tyrells. In the novels, Renly continued to use the traditional yellow-and-black sigil of House Baratheon after he declared himself king, but his TV counterpart adopts the green and gold of House Tyrell for his banner, which is Renly's way of demonstrating to Loras that he is indeed a Tyrell at heart if not in name, because Highgarden's colours are now his own. Loras' Freudian Slip in "Valar Morghulis" ("My sister Margaery, her husband was taken from us before...") proves that he considers Renly to be his "husband."
- Margaery attempts to invoke this with her dear friend Sansa Stark, by trying to set up a betrothal between Sansa and Loras. The Lannisters interfere before it can get very far, but the girls enjoy the idea while it lasts.
- In Friends:
- Ross witnesses Chandler having sex with his sister Monica and angrily declares "I can't believe it, my best friend and my sister!" When Chandler and Monica explain that they're in a serious relationship and in love and not just fooling around, Ross repeats it, only this time overjoyed. Of course, he was taking anti-rage drugs at the time.
- When Ross and Rachel start dating, Monica almost name-drops the trope, saying she and Rachel will be "friends-in-law".
- On Doctor Who, the Doctor marries River Song (a.k.a. Melody Pond, Amy and Rory's daughter from the future). Amy (who predicted the match a season and a half earlier) has a funny moment when she realizes that, as a result of it, she's now the Doctor's mother-in-law!
- Subverted with Jim and Andy in According to Jim: They didn't start off as friends but become this after Jim marries Andy's sister. Jim is also really close to his mother-in-law to the point where Andy thinks she prefers him as a son.
- In the pilot episode of the original Battlestar Galactica, Apollo and Starbuck were heading this direction when the latter was planning to be sealed to Athena but then broke it off with her following the Destruction of the Colonies. Apollo seems to understand this for the most part, knowing him...
- In the finale of Scrubs, JD pictures his son, Sam, and Turk's daughter, Izzy, all grown up and announcing their engagement to JD and Turk. Cue squeeing JD and Turk. Notable in that, unlike JD's other fantasies, this is part of a montage that JD and the finale strongly imply to be possible.
- In Princess Returning Pearl:
- Xiao Yan Zi marries her best friend/sworn sister Zi Wei's brother, Yong Qi. Yong Qi's good friend Er Kang also marries his sister Zi Wei.
- Then Xiao Jian, Xiao Yan Zi's brother and thus Yong Qi's brother-in-law, later marries Yong Qi's cousin.
- In the series finale of Will and Grace, Will and Grace themselves become this after their children marry.
- On Heroes, Hiro's best friend Ando has been in love with/crushing on his older sister Kimiko since they were kids. Thanks to some meddling in the past by Hiro, Ando and Kimiko are engaged and Hiro is absolutely delighted.
- Inverted in Breaking Bad; there's no indication that Hank and Walt were ever friends before marrying sisters Skyler and Marie, but by the time of the series, Walt has no closer male friend, and Hank's only other close male relationship is his partner Gomez. It makes it heartbreaking when they end up at odds, especially since Walt continues to consider Hank "family" and one of the few people he would never consider arranging a convenient "accident" for, even if the alternative is being caught or killed himself.
- Modern Family sets this up by its very nature:
- All twelve leads are immediate family and very close to all the others, so this happens whenever there's an in-law relationship. In particular, Phil and Mitchell, and Claire and Cam are siblings-in-law who enjoy spending time together independently of the rest of the family to pursue particular interests or hobbies. Phil and Cam might also count in this regard, since they're married to siblings from the same family and often hang out together.
- Jay shares this relationship with one son-in-law (Cam) but not the other (Phil). Poor Phil obviously feels snubbed by this, though it becomes increasingly apparent that Jay cares for him much more than he lets on; to be fair, he's often harsh to Cam too (particularly when referring to him as "my son's friend" in front of his buddies), and only really bonds with him over their shared love of sports.
- One of the reasons Phil is such a big Shipper on Deck for Haley and Andy is clearly that he would like nothing better than to have Andy as a son-in-law. The feeling is obviously mutual, as Andy idolises Phil.
- In an alternate timeline on Star Trek: Voyager, Harry Kim marries the (rapidly aged) daughter of Tom Paris and Kes.
Religion and Mythology
- In The Bible, not-yet-King David marries Michal and is best friends with her brother, Jonathan. To complicate matters, their father, Saul, wants to kill him.
- In Ys The Oath In Felghana, the Player Character's best friend (who's homeland/hometown the game's set in) mentions this trope in regards to his childhood best-friend Chester's sister, then notes that she'll probably end up with The Hero.
- The Elder Scrolls V: Skyrim has a potential case of at least friends in law: the Dragonborn can befriend Annekke Crag-Jumper by doing a quest for her (as indicated by it becoming possible to recruit Annekke as a follower). The Dragonborn can then make her this trope by marrying Sylgja, Annekke's daughter. The Dragonborn can also befriend Vilkas and Farkas, two members of the Companions guild who are fraternal twins and potential followers; both are candidates for marriage, and if the Dragonborn marries one, this can be implied with the other.
- Fire Emblem Jugdral:
- In the game, Quan is married to Sigurd's younger sister Ethlyn. The two of them have been friends (along with Eldigan) since their days at the military academy.
- In Generation 2, Sigurd's son Seliph can potentially end up with Lana, Larcei or Nanna (or their replacement characters Mana, Radney, or Janne). All three of them are the sisters of his respective childhood friends Lester, Ulster, and Delmud (or Dimna, Roddlevan, and Tristan). Larcei is also the cousin of Seliph's Big Brother Mentor, Shanan. Naturally, any possible pair up of this group will result in this. Nanna (who can also end up with Quan's son Leif) is also Eldigan's niece, so it's a double example. It's also possible to hook up the children of Azel and Lex or Alec and Noish. While it's a bit tricky, it's also possible to pair Seliph's half-sister Julia up with any of the possible male characters.
- In Fire Emblem: The Sacred Stones, Tana and Eirika are best friends whereas Innes and Ephraim are rivals. If Eirika marries Innes or Tana marries Ephraim...
- In Fire Emblem Awakening, the Avatar and Chrom are this if a male Avatar marries Lissa, Emmeryn, Lucina, or Cynthia or Kjelle if either of them are Chrom's daughter. A female Avatar and Chrom are this if she marries Brady or Inigo if either of them are Chrom's son. Lissa and Maribelle are this if Maribelle marries Chrom, and Chrom and Vaike can be this if Lissa marries Vaike. The indirect version is attainable if any of the Avatar's children marries one of Chrom's kids.
- The player character can become friends-in-law with Veronica, who more or less runs Oak Tree Town in Story of Seasons, by marrying her daughter Angela. In fact, in order for the romance with Angela to progress to the marital level, the player character must have a good friendship level with Veronica.
- Fire Emblem Fates: Due to time distortion shenanigans, the male Avatar can marry the daughter of their good friend Silas.
- Silas can also marry one of the male Avatar's sisters, Hinoka, Sakura, Elise, or Camilla.
- Roommates has Aisling clearly hoping for this between Sarah and her brother, Jareth. She's shown to be one of Sarah's friends in college and calmly foils Odile's stalking of Sarah on the grounds that, "She's going to be my sister".
- The Flintstones animated special, I Yabba-Dabba-Do, centered around Pebbles and Bamm-Bamm getting married, making the Flintstones and Rubbles in-laws. In the next special, Hollyrock-a-Bye Baby, Pebbles and Bamm-Bamm become parents after Pebbles gives birth to a pair of boy-girl fraternal twins they name Chip and Roxy.
- My Little Pony: Friendship Is Magic: Twilight Sparkle had Princess Cadence as a foal-sitter and considered her a friend, but they fell out of contact. The next she hears from Cadence is a wedding announcement—the groom is her older brother Shining Armor.
- Avatar: The Last Airbender and The Legend of Korra:
- Aang and Katara's son, Tenzin, was in a long-term relationship with Lin, the daughter of their friend Toph, but the pair broke up and Tenzin married someone else.
- Played straight with Aang marrying his friend Sokka's sister, Katara.
- The original series has version of this: Sozin and Roku were best friends early in their lives, and their grandchildren, Ozai and Ursa, wind up getting married. Though neither the friendship nor the marriage ended happily...
- On American Dad!, one of the only recurring Love Interests that Steve has is his friend Toshi's younger sister Akiko. His other friend, Snot, sometimes has a Precocious Crush on Hayley, though not much comes of that.
- A variation with Sun Ce and Zhou Yu, who marry the Qiao sisters.
- Wrestlers Barry Windham and Mike (I.R.S.) Rotundo
- Joaquin Phoenix and Casey Affleck (the latter is married to the former's baby sister).
- Jean Lannes' motivation for asking Napoleon Bonaparte's sister Caroline in marriage was not so much that he loved her, but because he wanted to be closer to the Corsican general he saw as one of his best friends. In the end she married another of her brother's generals, Joachim Murat, and they became king and queen of Naples. General Marmont inverted this: when Bonaparte offered to let him marry Pauline, another of his sisters, Marmont refused on the grounds that he was not genuinely in love with her. (Though when he saw the fate of the man Pauline eventually married, who died of yellow fever in Santo Domingo, he said in retrospect that he did not regret turning her down.)
- John of Gaunt and Geoffrey Chaucer, both members of the court of King Richard II of England and apparently close friends, were married to Katherine and Phillipa de Roet, respectively. Incidentally, this makes Chaucer the great-umpty-great uncle by marriage of every British monarch since Henry VII (a descendant of John of Gaunt and Katherine de Roet on his mother's side).