In fiction (not to mention RealLife), it's not uncommon for friends to be [[TrueCompanions as close as family]]. For the most part such relationships go unformalised. You might wish you could make a friend an official relative, but your options are limited. Adoption is difficult, especially when both people involved are adults. And marrying your friend probably isn't an option if you [[HeterosexualLifePartners just don't]] [[PlatonicLifePartners feel that way]] [[LikeBrotherAndSister about each other]].

But there is another option that some characters take: marry one of your friend's relatives! Or fix it so that one of your relatives marries one of theirs. Then you'll become in-laws, which is almost as good as being blood relatives, and gives you an excuse to spend vacations and holidays together. Some common variations:
* 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.

Often in these situations the romantic relationship comes off as secondary to the platonic one, with the spouse potentially being a SatelliteLoveInterest. Sometimes the platonic relationship may even be the primary motivator for the marriage, though it's hoped that most characters genuinely MarryForLove.

May overlap with SettleForSibling or DoubleInLawMarriage. Not to be confused with making friends of your in-laws.

Especially in modern times, when marriage rates are on the decline, a long-term non-married relationship can qualify for this trope, provided it brings two friends closer together and doesn't [[MySisterIsOffLimits create tension between them]].

----
!!Examples:

[[foldercontrol]]

[[folder: Anime And Manga ]]

* In ''Anime/CodeGeass'', Lelouch considers this a favourable outcome with his best friend/rival Suzaku and sister Nunally. [[spoiler: They might have hooked up in the end, but Lelouch isn't around to be an inlaw.]]

[[/folder]]

[[folder: Fan Works ]]

* In the ''Manga/DeathNote'' [[AlternateUniverseFic AU]] ''Fanfic/ThoseWhoStandForNothingFallForAnything'' 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 ''Franchise/HarryPotter'' [[AlternateUniverseFic AU]] ''[[FanFic/ThreeSlytherinMarauders 3 Slytherin Marauders]]'' Snape and Lucius become this when [[spoiler: Snape adopts Harry and Lucius marries Petunia and adopts Dudley.]]
[[/folder]]

[[folder: Film - Live Action ]]

* At the end of the original ''Franchise/StarWars'' trilogy, Luke Skywalker and Han Solo seem to be headed in this direction.
* Might just happen in the ''Fast and the Furious'' film series, with Brian and Dom's sister, Mia.
* In ''Film/NottingHill'', 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.

[[/folder]]

[[folder: Literature ]]

* At the end of the ''Literature/HarryPotter'' series, Harry becomes [[spoiler:Ron's brother-in-law]] by marrying [[spoiler:Ginny Weasley]]. Since [[spoiler:Hermione marries Ron]], he also effectively becomes [[spoiler:Hermione's brother-in-law]].
* In the appendices to ''Literature/TheLordOfTheRings'', Sam's daughter Goldilocks marries Pippin's son Faramir.
* In the ''Franchise/StarWarsExpandedUniverse'', this trope becomes canon for Luke and Han Solo.
* No one comes out and says it, but at the end of the ''Literature/{{Belgariad}}'' Durnik, Garion's oldest friend, technically becomes his uncle by marrying Polgara.
* The backstory to ''Literature/ASongOfIceAndFire A Game of Thrones]]'' 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 Ned's sister Lyanna. That fell through when Rhaegar Targaryen kidnapped her, leading to Robert's Rebellion and her death during it.
** Robert tried to invoke this trope by arranging marriage between his oldest son and Ned's daughter Sansa, although a more cynical possible take on this is that he offered the marriage to buy Ned's service as his chancellor.
* At the end of ''Literature/PrideAndPrejudice'', Elizabeth marries Darcy and her sister Jane marries his best friend Bingley.
* In the ''Literature/LittleWomen'' series, Jo [[spoiler:names her son after]] her sister's husband.
* In ''Literature/TheBigWave'', Kino and Jiya are childhood friends, and Jiya grows up to marry Kino's sister, Setsu.
* {{Literature/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. [[spoiler: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 ''Literature/{{Overenskommelser}}'' by Creator/SimonaAhrnstedt, 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 [[spoiler:her gay lover Venus]].
[[/folder]]

[[folder: Live Action TV ]]

* ''Series/GameOfThrones'': 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 brother-in-laws 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' FreudianSlip in "Valar Morghulis" [[spoiler:("My sister Margaery, her husband was taken from ''us'' before...")]] proves that he considers Renly to be his "husband."
* In ''Series/{{Friends}}'' Ross discovers that Chandler's now in a relationship with his sister Monica and angrily declares [[MySisterIsOffLimits "I can't believe it, my best friend and my sister!"]] When Chandler explains that they're in love and not just fooling around, Ross repeats it, only this time overjoyed. Of course, [[MoodSwinger he was taking anti-rage drugs at the time]].
* On ''Series/DoctorWho'', [[spoiler:the Doctor]] marries [[spoiler:River Song]] (a.k.a. [[spoiler:Melody Pond, Amy and Rory's daughter from the future]]). [[spoiler: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 [[spoiler:the Doctor]]'s mother-in-law!
* Subverted with Jim and Andy in ''Series/AccordingToJim'': 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 ''Series/{{Battlestar Galactica|Classic}}'', 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, [[TheCasanova knowing him]]...
* In the finale of ''Series/{{Scrubs}}'', JD pictures his son, Sam, and Turk's daughter, Izzy, all grown up and announcing their engagement to JD and Turk. Cue [[{{Squee}} squeeing]] JD and Turk. Notable in that, [[ImagineSpot unlike JD's other fantasies]], this is part of a montage that JD and the finale strongly imply to be possible.
** Dr. Cox and Ben are probably an inversion, due to Cox's divorce, but nonetheless they were best friends. [[spoiler:Cox took it very hard when Ben died of leukemia.]]
* In ''Series/PrincessReturningPearl'', 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 ''Series/WillAndGrace'', [[spoiler:Will and Grace themselves become this]] after their children marry.
* On ''Series/{{Heroes}}'', Hiro's best friend Ando has been in love/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.
[[/folder]]

[[folder: Video Games ]]

* In ''[[VideoGame/{{Ys}} Ys The Oath In Felghana]]'', the PlayerCharacter'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 [[GenreSavvy notes that she'll probably end up with]] TheHero.
* ''VideoGame/TheElderScrollsVSkyrim'' has a potential case of at least ''friends'' in law: the [[PlayerCharacter 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 also marry Sylgja, Annekke's daughter.
** The Dragonborn can also befriend Vilkas and Farkas, two members of the Companions guild who are identical twins and potential followers; both are candidates for marriage, and if the Dragonborn marries one, this can be implied with the other.
* In ''VideoGame/FireEmblemAwakening'', the Avatar and Chrom are this if a male Avatar marries Lissa, [[spoiler: Emmeryn, Lucina, or Cynthia or Kjelle if either of them are Chrom's daughter]]. A female Avatar and Chrom are this if she marries [[spoiler: 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 [[spoiler: any of the Avatar's children marries one of Chrom's kids]].

[[/folder]]

[[folder: Western Animation ]]

* ''WesternAnimation/TheFlintstones'' animated special, ''I Yabba-Dabba-Do'', centered around Pebbles and Bamm-Bamm getting married, making the Flintstones and Rubbles in-laws.
** [[spoiler:In the next special, ''Hollyrock-a-Bye Baby'', Pebbles and Bamm-Bamm become parents after Pebbles gives birth to a pair of [[UntoUsASonAndDaughterAreBorn boy-girl fraternal twins]] they name Chip and Roxy]].
* ''WesternAnimation/MyLittlePonyFriendshipIsMagic'': 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.

[[/folder]]

[[folder: Real Life]]

* A variation with Sun Ce and Zhou Yu, who marry the Qiao sisters.
* Wrestlers Barry Windham and Mike (I.R.S.) Rotundo
* JoaquinPhoenix and Casey Affleck (the latter is married to the former's baby sister).
* Jean Lannes' motivation for asking NapoleonBonaparte'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. Another of his comrades, Marmont, arguably one of Bonaparte's best friends in their early years, 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 Creator/GeoffreyChaucer, 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 [[TheHouseOfTudor Henry VII]] (a descendant of John of Gaunt and Katherine de Roet on his mother's side).

[[/folder]]

----