Asu no Yoichi! has a variant of this (Ayame likes Youichi who is kind of a pervert but seems to prefer Ibuki)
School Rumble has a one-sided variant: Yakumo likes Harima who likes Yakumo's sister Tenma.
RahXephon has a more serious example with Megumi and Haruka both liking Ayato. Megumi later decides to back off.
In fact, there's a recursive example (two of the characters are in both love triangles) kinda, sorta with Dr. Kisaragi and Ayato, who are clones, both loving Haruka
Infinite Ryvius spoiler:This is the origin of the Cain and Abel feud of Yuki and Kouji Aiba; they had both fallen for the same woman (implied to be Aoi), and Kouji bows out for both their sakes when he finds out. Yuki takes that as an insult to the woman, and his disgust for his brother lasts for most of the series.
At one point in the D.N.Angel anime, both Risa and Riku are in love with Niwa. (With the added complication of Dark being in love with... One of them. It's complicated.)
Chocolat and Tira Misu for Carrot Glace in Sorcerer Hunters. Tira wins in the manga.
In Spiral, Ayumu was in love with his sister-in-law, and lives with her after his brother disappears.
In Adachi Mitsuru's Touch, twin brothers Katsuya and Tatsuya both love their childhood friend Minami. Older twin Tatsuya backs off, allowing the younger Kazuya to win at everything, including love seemingly. Then... Kazuya dies in an accident, and the story follows the relationship that develops after this tragedy.
In Ouran High School Host Club, manga version, both Hikaru and Kaoru are in love with Haruhi. Kaoru backs out because "Hikaru's more important to [him]".
This also starts off the plot of Judge: Hiro has a secret crush on his friend Hikari, but his older brother Atsuya confessed his own attraction first, so he kept quiet about it. However, at the start of the series, he decides to tell her anyway, and secretly derails the pair's plans to meet for a date, figuring she might be more receptive to his confession if she thought Atsuya had stood her up. Atsuya gets hit by a car because of this.
Onegai Twins plays out like one of these. The plot consists out of two girls that turn up at a boy's house claiming to be his twin sister. The boy and the girls need to find out who is the real twin sister while falling in love with each other.
Wandering Son initially had a Love Triangle between two siblings. Maho fell for a boy named Riku, so she asked her brother Shuichi to dress up as a girl after Riku had mistaken him for a girl and fallen for him. Even after he finds out, he seems to be somewhat attracted to Nitori; it's faded though, and he and Maho are a happy couple. It's never quite said whether Shuichi herself had feelings for Riku.
In Star Driver pink-haired twin sisters Mizuno and Marino You both fall for the same boy, male lead TakutoTsunashi. It is later revealed that Marino is in fact a clone created by Mizuno's first phase power, which may account for them both liking 'the same things.'
A variation of this can be found in The World God Only Knows, with the Jupiter Sisters, who possess the bodies of girls belonging to Keima's Unwanted Harem. Somewhat subverted later on, after one of them revealed that they're not actually related and the "Jupiter Sisters" is a collective title bestowed to six priestesses whose powers complement each other to a devastating synergistic effect. They truly regard of each other as sisters, though.
Koki, Tanpopo, and Yoji in Imadoki It's more one-sided on Tanpopo's end, as she and Koki very clearly have feelings for each other, but when Yoji confesses his own feelings for Tanpopo, she briefly dates him.
Claudine has two. Both of the Lacques siblings (Louis and Cecilia) were lovers to the main character's father Auguste, and later both Claudine and his elder brother Andre fall for the same girl, Sirene. Both cases end up in death and tears.
Played with in Anatolia Story. Yuri's twin caretakers Ryuhi and Shala have the same taste in men, thus they fall for Kail's advisor Kikkuri and visit him at night, with Kikkuri not realizing that he's sleeping with them both since he can't distinguish them. When the girls get pregnant at the same time and the deal is revealed... Kikkuri marries them both, and the seven members of their little family (since both girls gave birth to twins) live happily from then on.
Happens in Yoko Hanabusa's Lady, with the Russell half-sisters Sarah and Lynn having feelings for Arthur Brighton. Sarah and Arthur get together, while Lynn's feelings shift towards Arthur's younger brother Edward.
Fullmetal Alchemist: Apparently, Ed & Al used to have this in their childhood, fighting over who would marry their friend Winry, who wasn't interested in either of them. Eventually, though, Winry did develop a thing for Ed, and Al gave up on her, instead falling into the role of Shipper on Deck.
Sakura Trick: Haruka has the affections of both Sonoda sisters. Since her relationship with the younger one is the entire basis of the series, it's easy to guess how things will turn.
A good portion of Princess Jellyfish's plot revolves around two brothers both liking the same woman, one being a Wholesome Crossdresser and the other not even realizing the woman is the same person as the geeky, meganekko he hates.
In Buffy the Vampire SlayerSeason Eight, Buffy realizes that she may have romantic feelings for long-time platonic friend Xander. She goes to tell him, only to discover that he has just started a relationship with her sister, Dawn. (This is all made even more complicated when you remember that Xander had a crush on Buffy in season one and Dawn had a Precocious Crush on Xander in season five).
In one BatmanBlack and White animated comic "Two of a Kind", Harvey Dent is rehabilitated by a combination of therapy, plastic surgery, and a romance with the plastic surgeon. Said plastic surgeon has a twin sister who is immediately smitten with Harvey because she is even crazier than Two-Face and believes they are two of a kind. She tempts Harvey into cheating on his fiance, but he eventually breaks it off with her. It ends in tears just as all of Harvey's attemptsto be free of Two-Face do.
Issue #9 of My Little Pony Friends Forever involves the identical twin unicorn Con Artists Flim and Flam having broken up over their mutual attraction to a librarian named Marian. Granny Smith gets them back together by revealing she was the non-sibling member of one of these herself in her younger days, with the "Jonagold Brothers" never speaking to each other for the rest of their lives after she backed out of their feuding over her.
Cinderella's stepsisters have been known to cut off parts of their feet to fit in the shoe.
In "The Wonderful Birch", the witch made the items fit her daughter; her stepdaughter let the prince know the truth and so they used the witch's daughter to make a magical bridge and he married the stepdaughter; the witch then replaced the stepdaughter with her daughter after childbirth.
In the Chinese tale "Beauty and Pockface", Pockface takes Beauty's place after she visited her family, and weeps and wails about how he never really loved her because he no longer does after she contracted smallpox.
In "Fair, Brown, and Trembling", Trembling goes to church, and a prince falls in love with her, and out of love with Fair. After she has a baby, Fair pushes Trembling into the sea and takes her place.
The Patrick Dempsey film Some Girls has a young Dempsey falling for three sisters, although he sleeps with only one and in the end doesn't wind up with any of them.
The Emperor murdered his brother and married his brother's wife in Legend Of The Black Scorpion. Wan didn't want anything to do with this, but she submits. Or appears to.
Implied in The Lone Ranger with Dan and John with Rebecca before John left to learn law. Dan married Rebecca and have a son, Danny Reid (Jr.). It is implied that Dan knows John and Rebecca still loves each other.
Linda and Julia Seton get into one when Linda falls in love with Julia's fiancé Johnny in Holiday. Linda, who loves her sister, staunchly refuses to act on her feelings until the end of the movie.
Silent comedy film Hands Up! features the protagonist falling in love with each of a pair of sisters. He resolves this dilemma by—marrying both of them.
In I Wake Up Screaming, Frankie the promoter takes an interest in Vicki, plucks her from waiting tables, and makes her a famous model—but winds up falling in love with her sister Jill, much to Vicki's displeasure.
In Harry Harrison's The Stainless Steel Rat Goes to Hell Digriz' two twins fall in love with the same woman. The problem is solved by going to a parallel universe and fetching an alternate version of said woman.
Sandy and Dennys in Many Waters both love Yalith and she loves them both back. Unfortunately, the relationship as such is doomed because the flood's coming and Yalith isn't allowed on Noah's ark.
Played with darkly in Hellraiser: Julia has married Larry, but eventually murders him in preference for his brother Frank, who seduced her prior to the events of the film. It doesn't really go well for her....
The Bible has a rather complicated example with Jacob, Rachel and Leah. Jacob fell madly in love with Rachel, and worked for her father for seven years in order to earn the right to marry her. However, in Rachel's society it was shameful for the younger daughter to be married first, so (with some help from the very thick veils used at the time, as well as some alchohol) their father switched Rachel out with Leah, with Jacob only finding out during the wedding night. Rachel and Leah's father then forced Jacob to work another seven years so he could marry Rachel for real. One can't help but wonder what poor Leah felt about this arrangement. (In a slightly unrelated note, this is why we have the tradition of the bride removing her veil in Western weddings.)
For an added complication, a Jewish legend says that Leah was originally destined to marry Jacob's brother, Esau, who was far less meritorious; Leah was overwrought and prayed to God, who then arranged things so she could marry Jacob as well.
And, to make things even more complicated, two handmaidens were involved. Who in some legends were also Leah and Rachel's sisters.
In the biblical fiction book The Red Tent, about Jacob and Leah's daughter Dinah, this is one of the main plot points. Jacob loves Rachel but also loves Leah. This is further complicated by the book saying that the handmaidens (Bilhah and Zilpah) are their sisters. Which is true in some Jewish lore as well.
In the Isaac Asimov short story To Your Health, two brother opticians are in love with the same eye-fixated girl.
The four main characters of The Demon's Lexicon are two sets of siblings. Of each set, one sibling is attracted to both of the other siblings. Only one of these attractions is Played for Drama; the other fizzles out as the party instead develops a crush on the Big Bad.
East of Eden (which is, after all, based on The Bible), has two of these. Cathy marries Adam after sleeping with his brother Charles. Later, Cathy's sons Cal and Aron like the same girl, Abra. She's interested in Aron when they're children, but their relationship becomes strained in high school, and she switches to Cal toward the end of the book.
In the backstory of Eight Cousins, a woman named Rose was courted by the brothers George and Alec Campbell. She chose George and married him, but at the cost of the distancement between George and and Alec. Sadly, Rose died early in her life, and some years later George perished as well; in George's deathbed, the brothers were reconciled, and Alec promised the dying George to take care of his and Rose's daughter, Rose Jr. aka the female lead of the story.
In One Hundred Years of Solitude, Hookerwith A Heart Of Gold Petra Cotes regularly sleeps with the twins Jose Arcadio Segundo and Aureliano Segundo without realizing they're not the same dude. The three contract a STD, which reveals the whole deal, and she breaks up with them. Jose Arcadio Segundo leaves Petra after getting better, but Aureliano Segundo gets her forgiveness and keeps her as The Mistress since he cannot marry her.
In the Lowlands Of Scotland series, sisters Rose and Leona both fall for their cousin Jamie after he arrives at their father's home. However, Jamie only has eyes for Rose, until he accidentally ends up married to Leona and learns to love her. And then when Rose's machinations work and he is broken up from Leona and married to Rose, he has to learn to love Rose all over again. And the sisters have to work through both of them loving and having been married to the same man.
Also averted in Eva Luna. Rolf Carle is smitten with both of his cute cousins and cannot decide who to choose since both girls are appealing to him in their own ways. The girls talk about this and they decide to... share his body and affection and tackle him together when he's working in the garden. They keep this relationship for several years, until the girls marry local guys and Rolf becomes an Intrepid Reporter; nevertheless, they always remember these times very fondly.
Played with in The Infernal Devices. Though the main character, Tessa's, love interests are not biological brothers, they are parabati, which for Shadowhunters is quite close to being brothers. There isn't a 'Mundane' equivalent for it.
Live Action TV
Arrested Development played with this. Michael develops feelings for Marta who is with GOB. GOB is mostly using her but he doesn't want Michael to have her either. Also Buster has a crush on Marta. All three of them are rebuffed by Marta eventually.
Later there's a Running Gag of GOB trying to seduce women that Michael likes, but repeatedly getting the wrong woman. (For instance, when Michael is dating his son's ethics teacher, GOB ends up seducing a different teacher by mistake.)
Tommy and Justin of Brothers and Sisters were both dating Lena, though not precisely at the same time.
In the British thriller series The Last Enemy, Stephen and Yasim start a relationship with each other after her husband (his brother) dies in a car bomb. Then the brother comes back.
The Vampire Diaries brings us Damon and Stefan Salvatore who became vampires after falling in love with Katherine, a vampire that abuses their love triangle. After she is seemingly killed the brothers eventually end up in Mystic Falls where they both fall in love with Elena, who looks exactly like Katherine.
And in the past, Klaus and Elijah over Katherine.
Klaus reveals that he and Elijah fell in love with the same girl - Elena and Katherine's ultimate ancestor - when they were still human. It was her blood that they unknowingly consumed when their mother turned them into vampires, which explains the need for the doppelganger. (Katherine is the first doppelganger and Elena is the second).
While they aren't siblings Elena/Stefan/Katherine kinda counts considering that Katherine is Elena's great-something grandmother. Try not to think about that one too hard.
A weirder example is the Declan/Holly J/Fiona triangle. Because they moved around a lot Declan and Fiona were very close but after Declan fell for Holly J, Fiona did everything she could to try to drive Holly J away including kissing her brother at a party. After getting therapy, Fiona became good friends with Holly J eventually realized she’s a lesbian and was in love with Holly J.
One episode of Roseanne has Mark almost take a girl out on a date just because his younger brother David wanted to. His Jerk Ass-ery is repaid when his wife almost finds out, and David, after helping him hide it from her, asks Mark if he really wants to go out of his way to alienate the only two people who really care about him.
One episode of Sister Sister has a guy accidentally ask identical twins Tia and Tamara out on a date because he couldn't tell them apart. The three go on a rather awkward date together when he comes over and the mistake is realized. Both also liked Roger after he suddenly becomes hotter, and he winds up dating each of them at some point.
In an episode of Frasier, Frasier and Niles think they're participating in this trope as they compete for the affections of a certain woman at a party, but they realize they've been talking about two different women all night...right after they decide their sibling bond is more important than a random woman, let the object(s) of their affections leave the party without propositioning them. Cue double Oh Crap expressions.
In The Borgias, thirteen-year-old Joffre Borgia marries Femme Fatale Sancia, who has a torrid affair with his elder brother Juan. Incidentally, Juan was supposed to be the one marrying Sancia in the first place but he refused on the grounds that she's a mere Royal Bastard, (despite the fact that he's infamously illigitimate himself what with being the son of the Pope and all) but he doesn't mind having sex with her literally seconds before she consummates her marriage with poor Joffre. Also, while the show hasn't had time to portray this yet, historically Sancia later had an affair with the eldest Borgia sibling Cesare, making it a sibling love square (this situation is sometimes cited as the reason for Juan's eventual murder at the hand of Cesare.) Yeah. The term Big Screwed-Up Family barely even covers it. Particularly when you factor in the various Sibling Triangles engendered by Cesare and Lucrezia’s incestuous relationship (whether the show will actually go there remains to be seen, but so far the Incest Subtext is hardly subtle). Cesare threatened to murder her first husband Giovanni if he ever hurt her, and he allegedly DID murder her second husband Alfonso (Sancia’s brother) purely out of jealousy.
Wataru, Taiga and Mio in Kamen Rider Kiva, though Wataru and Taiga initially do not know they are brothers.
On Murdoch Mysteries, Detective William Murdoch and Dr. Julia Ogden have the cutest Will They or Won't They?tension. In the episode "Houdini Whodunit", Julia's younger sister Rugy appears and flirts with Murdoch immensely daringly, and even invites him for romantic dinner, which is quite a big deal for the Victorian times. However, Julia doesn't give up: When she comes to the restaurant to discuss the case (originally unaware that they are there together), she seems hurt by seeing them, but she ultimately decides to join them and fight. Understandably, Ruby is irritated, but when she sees how serious Julia is about the Detective, she starts supporting her and becomes their Shipper on Deck.
On Grimm, the Renard half-brothers have both had involvements with Adalind and it's currently uncertain which, if either, is the father of her unborn child. She may not even be all that sure about that point herself.
The Brady Bunch episode "Cyrano de Brady" has Peter pining for a friend of Jan's and asks for Greg's help in wooing her. Unfortunately, the girl thinks it's the other way around and throws herself at Greg, pissing Peter off and leaving Greg with an unwanted admirer. Fortunately the triangle works in Peter's favor in the end.
A smaller one occurs in "The Not-So-Ugly Duckling", when a boy Jan likes drools over Marcia. However, Marcia has zero interest in the boy, despite Jan's accusations of boyfriend-stealing.
In Helix, Julia, a CDC researcher, became unhappy in her marriage to coworker Alan and turned to his younger brother Peter, a research scientist, for comfort. After Alan caught them in bed together, Julia divorced Alan and totally cut off contact with Peter. In the present, both brothers carry a torch for Julia, while Julia struggles with unhappy memories of her marriage to Alan and still-present feelings for Peter, all in the midst of an outbreak of The Virus at a Research, Inc., which Alan and Julia have been sent to contain, with Peter as the Patient Zero.
Arrow has the Lance sisters, Sarah and Laurel who competed, rather bitterly, for Olliver's affections. Sara being on the boat with Olliver has shades of revenge and spite for Laurel.
In the CSI: Crime Scene Investigation episode "Exile", two sisters loved the same man who had been a childhood friend, but he only reciprocated the feelings of one. The episode begins with one sister's body being discovered and it goes downhill from there.
In the folk song "William and Davy", twins William and Davy fight bitterly over the woman they're both in love with... who, as it turns out, isn't interested in either of them.
At the end of November 2010, both of the Bella Twins started going after Daniel Bryan in WWE Raw, with this trope being the inevitable result.
Verdi's Il Trovatore. And the boys don't even know they're brothers. Consequence? Well, it's an opera...
Other operatic examples include Pelléas et Mélisande by Debussy (based on Maeterlink's play), and Amadis by Jules Massenet.
A variation exists in The Love of the Nightingale. Tereus has an Arranged Marriage to Procne but falls for her sister Philomele. However neither even likes him.
This is basically the plot of "The Bartered Bride", the opera by Bedrich Smetana. Marenka and Jenik want to get married, but Marenka's parents have already employed a matchmaker to hook her up with the son of a rich man named Micha. The matchmaker demands Jenik break up with Marenka, and Jenik promises he will do so if the matchmaker pays him off and promises that Marenka won't marry anyone else but Micha's son. Jenik, of course, turns out to be Micha's long-lost son, and he and Marenka get married.
A major plot point in Blood Brothers is both Mickey and Eddie being in love with Linda. However none of the three know that Mickey and Eddie are brothers.
In Hamlet, Claudius murders his brother Hamlet Sr. and marries his brother's wife Gertrude a month later. It's heavily implied that she herself was in on the plot.
In the Warcraft universe, Night Elf brothers Malfurion and Illidan Stormrage both vie for Tyrande Whisperwind's affections. Illidan's rage and jealousy towards Malfurion for so effortlessly (and unintentionally) winning Tyrande's affections played a large part in triggering Illidan's Face-Heel Turn.
In Persona 2: Eternal Punishment, Tatsuya and Katsuya are both in love with Maya. Katsuya's Shadow taunts him with this and uses it to try to get him to shoot Tatsuya. Katsuya says that while the Shadow is right about his feelings, he loves his brother anyway, and shoots the Shadow instead.
In Dragon Age II, Hawke's younger brother Carver cautiously tries to win the affection of Merrill, one of Hawke's possible love interests. He does not succeed, as Merrill seemingly doesn't notice, but if Hawke and Merrill become a couple, in a later Party Banter Carver bitterly comments "So, you and Merrill.", to which an aggressive Hawke replies "Yes. I and Merrill". A similar situation can arise regarding Hawke's sister Bethany and Sebastian, but it is less severe, as Bethany and Sebastian don't spend much time in the party together.
Also implied that Merrill knows exactly where Carver's trying to go, but is feigning obliviousness to let him down gently, since she's got a massive crush on Hawke.
Carver also shares a mutual attraction with Isabela, which leads to a funny reaction from Hawke when Isabela gets turned on by Carver's flirtation.
In the backstory of Symphonic Rain, both Arietta and Torta Fine were in love with Chris and despite everyone saying Torta complimented him, he ended up getting together with Arietta.
In Taketo's route of Class Trip Crush, Taketo and the protagonist are well on their way to falling for one another when the protagonist finds herself hitting it off with Taketo's twin brother Yasuto... who quickly starts to fall for her as well. Confusion, awkwardness, and hurt feelings soon abound.
In the original Double Dragon, Jimmy Lee is in love with brother's girlfriend, Marian, who gets kidnapped by the Black Warriors. If two players defeat the final boss together, Billy will challenge Jimmy over Marian.
The twins Itsuki and Sayo both fall in love with Akira in Suika. It ends when right before Akira has to move away, they have a fight over who gets to give him the present they made for him. Itsuki pushes Sayo down the stairs and thinks she's killed her, before REALLY getting killed by her insane father. The Itsuki we meet later is actually Dead All Along, while Sayo is hospitalised and in a coma.
Semi-example: In one episode of Teenage Mutant Ninja Turtles, three of the four Turtles eat pizza tainted with a love potion, causing those three to fall in love with Irma. Hilarity and in-fighting ensue, until Splinter and the unaffected Turtle, Donatello, find a way to counteract the potion. Afterwards, further hilarity ensues when Irma discovers who was actually courting her.
Subverted in the end of season 2 where Korra and Mako decide to break up and stay as Just Friends, and Bolin is recovering after settling things with his Yandere ex-girlfriend Eska (Who happens to be Korra's cousin).
Happens in Gargoyles, though this is an odd example since all gargoyles of the same clan and generation consider themselves siblings. Still, "Othello" and "Iago" refer to each other this way as they fight over the former's mate, and all three members of the Trio compete over Angela when she first arrives.