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In fiction (not to mention Real Life), it's not uncommon for friends to be 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 just don't feel that way 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. Sometimes the platonic relationship may even be the primary motivator for the marriage, though it's hoped that most characters genuinely Marry for Love.
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 create tension between them.
- In Code Geass, Lelouch considers this a favourable outcome with his best friend Suzaku and his sister Nunnally: he wants him as a part of his plans and her to be protected by a huge Badass. They might have hooked up in the end, but Lelouch isn't around to be an inlaw.
- In the second part of Kyojin no Hoshi, the Friendly Rivals Hyuuma Hoshi and Mitsuru Hanagata find themselves in this position after the first's sister (Akiko) marries the latter. It's even lampshaded: when Hyuuma sees Mitsuru for the first time after a long time and calls him by name, the other man replies something by the lines of "But I'm your 'older brother'!"
- Takes place in Fruits Basket, when Yuki Sohma hooks up with his best friend Kakeru Manabe's younger half-sister, Machi Kuragi.
Film - Live Action
- At the end of the original Star Wars trilogy, Luke Skywalker and Han Solo are headed in this direction: Han is Luke's Big Brother Mentor, and at the same time his Belligerent Sexual Tension with Luke's sister Leia has gone the Relationship Upgrade way. The new movies 100% confirm it.
- At the end of the Harry Potter series, Harry becomes Ron's brother-in-law by marrying Ginny Weasley. Since Hermione marries Ron, he also effectively becomes Hermione's brother-in-law.
- 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 also becomes canon for Luke and Han.
- 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.
- Invoked in Friends when Ross discovers that his best friend Chandler's now in a relationship with his younger sister Monica and initially sees it as a case of My Sister Is Off-Limits. (To be fair, Ross found out about them in a very... awkward manner.)
Ross: (angry) I can't believe it, my best friend and my sister!
Chandler explains that it's a serious relationship.
Ross: (overjoyed) I can't believe it, my best friend and my sister!
- Doctor Who, when the Doctor marries River, leading to a funny moment when Amy realizes that 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 Tanked, Brett's best friend and business partner Wayde is married to Brett's sister Heather (the accountant of the aquarium-making Family Business). It even gets lampshaded in the opening of the show.
- 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.
- Fire Emblem has many of these:
- In the Akaneia games, Marth and Merric are Childhood Friends and Merric is the closest to a Love Interest in-story for Marth's older sister Elice.
- In Genealogy of the Holy War, Quan is Happily Married to his best friend Sigurd's younger sister Ethlyn. In the second generation, Seliph can hook up with either Lester's sister Lana, Ulster's sister Larcei (also the cousin of his Big Brother Mentor Shannan), or Diarmuid's sister Nanna (who is also the niece of Sigurd's other best friend Eldigan).
- In Fire Emblem the Sacred Stones, since Eirika has a Romantic Two-Girl Friendship with Tana and Eirika's twin brother Ephraim has a Friendly Rivalry with Tana's big brother Innes, this can take place if Ephraim marries Tana or Eirika marries Innes. Or both.
- This will happen in Fire Emblem Awakening, if a Male Avatar marries Chrom's younger sister Lissa. Or his older sister Emmeryn (if recruited), his Kid From the Future Lucina, or a Kjelle or a Cynthia fathered by him.; or if a Female Avatar marries a Chrom-fathered Inigo or Brady.
- In Fire Emblem Fates, this can happen if either the Male Avatar marries Silas' daughter Sophie, or Silas marries the Avatar of either gender's sisters (Sakura, Elise, Camilla or Hinoka).
- In Fire Emblem: Three Houses, if Raphael Kirsten and Ignatz Victor reach an A support, Ignatz marries Raphael's younger sister Maya. Similarly, if Raphael has an A support with Lysithea von Ordelia, it's said that first Lysithea and Maya became besties and then she's implied to have hooked up with Raphael. If Balthus von Albrecht marries Hilda Valentine Goneril, this takes place (off-screen) between him and her older brother Holst. Finally, if Jeritza von Hrym aka Emile von Bartels marries Constance von Nuvelle, Constance and Jeritza's older sister Mercedes will become this.
- In Guilty Gear game Revelator -SIGN-, something that lotsa fans speculated for years is confirmed: that Frederick/Sol Badguy is Dizzy's father.. This happens when Dizzy is all but married to Ky Kiske, who happens to be Vitriolic Best Buds with Sol. And, by that point, they already have a kid (Sin, the local Idiot Hero).. When both men properly realize that THIS is their technical "bond", Hilarity Ensues.