You have a younger sibling. Either full-blooded, half-blooded, or just adoptive (or, in some odd cases, more than one of those at once). Especially a little sister. The kid is wronged somehow. What do you do?
Do you deal with the offender in a calm, civil manner?
Or failing that, perhaps you are a little upset, yet you keep control over your emotions, and keep yourself as restrained as you can?
Or do you go after the offender, the offender's friends, anyone connected to the offender, as well as anyone who even looks like they could be an offender sometime in the indeterminate future and kill them all in as painful a manner as possible?
If you are the latter, then congratulations! You are officially a Knight Templar Big Brother (or Knight Templar Big Sister).
This trope is what happens when a character takes the Big Brother Instinct to the extreme. They don't just protect the younger sibling, they absolutely destroy anyone who'd so much as even look at them wrong. Much like the Knight Templar and Knight Templar Parent, the Knight Templar Big Brother often genuinely believes that what they are doing is right and fully beneficial to the younger sibling, which often results in a clash of siblings later on within the show or work.
Contrast with Spartan Sibling
See also: Big Brother Instinct, Knight Templar, Knight Templar Parent, My Sister Is Off Limits. If the characters aren't actual siblings, it probably is Yandere.
Mikasa Ackermann towards her adoptive brother Eren Yeager. She's violently over-protective, to the extent that she will ignore direct orders or the greater good (such as helping evacuate citizens) to protect him. On numerous occasions, she's threatened the lives of even her True Companions, making it clear if forced to choose between Eren's well-being and their lives, she will kill them without hesitation.
Levi was one to his ill-fated surrogate little sister, Isabel, in the Spin-Off prequel about his past. When she came home beaten up and missing a pigtail, he went out. He comes home that evening, cleaning blood off a knife and won't answer when Farlan asks if he killed those responsible.
Lelouch Lamperouge from Code Geass. After his younger sister Nunally was wounded in a terrorist attack (that also killed their mother), he started a war and took over the world, all for her sake. And when she found out, she gotpissed.
In Season 1 Mao, a Stalker with a Crush to Lelouch's partner C.C, kidnapped poor Nunnally and strapped a bomb to her. Lelouch was already very pissed off at Mao, since he first Mind Raped his poor, sweet classmate — and a total innocent bystander which he had no business to involve — Shirley and then did the same to his Forgotten Childhood Friend Suzaku; needless to say, Mao did not meet a pleasing end.
Ichigo adamantly believes the reason a big brother is born first is to protect all the little siblings that follow. He will force-feed this belief by the sword if he has to to any big brother he catches not protecting their own siblings, as both Orihime and Rukia's respective big brothers discover. And when he feels his own sisters are threatened and murder is the first thing he comes up with. And when Yuzu dies in fourth movie, due to being exposed to hell miasma for too long, he uttery flips out, goes into his super Hollow form and destroys Hell.
Once he manages to resolve his conflicting vows, Byakuya goes from being an Aloof Big Brother to a Knight Templar Big Brother where his little sister, Rukia, is concerned.
Toshiro Hitsugaya and Momo Hinamori have been raised together (the character book of Souls also confirms their relationship is "familial" and the old woman who raised them is regarded by both as their grandmother). He is incredibly protective of her despite being the younger of the two to the point where he attacked a perceived threat to her with such force two nearby vice-captains were caught up in the power he unleashed (one of which was actually her). In the end, their relationship is exploited by the Big Bad in one of the biggest Kick the Dog moments in the entire manga, the consequences of which is him making a vow to become strong enough to protect her from everything.
Henry Wong from Digimon Tamers went from being the most rational and calm guy in the group to being temperamental, hasty and reckless when his sister ends up in the Digital World.
Both Keiichi Maebara and later, Shion Sonozaki, from Higurashi no Naku Koro ni fall prey to this syndrome when their non-blood-related "sister", little Satoko Houjou, is abusedby her uncle. Winds up leading to two of the most tragic endings of the series.
It is heavily implied that her actual brother, Satoshi did the same regarding the aunt.
Caren, the Aloof Ally of Mermaid Melody Pichi Pichi Pitch, starts dangerously close to this, having been perfectly fine until her twin sister was kidnapped. However, while she blames and berates the good guys as well as the bad, she does do a lot of reluctant rescues and admits that she was wrong instead of going off the deep end.
Another Knight Templar Younger Brother is Fire God Apollo from Cyborg 009, towards his older sister Hunt Goddess Artemis. When the girl died very tragically, the already unstable Apollo went literally insane with grief and killed the culprit, then let himself die whispering his sister's name.
Ran Fujimiya in Weiß Kreuz is downright obsessively protective of his Ill Girl little sister Aya. He became an assassin in large part to pay her medical bills (she's in a coma), and he is not in the least bit rational about threats to her safety.
Kankuro becomes one of these as a part of his Character Development, reacting violently to anyone who badmouths his little brother Gaara in front of him and going after Gaara alone after Deidara and Sasori of Akatsuki kidnap him and Kankuro's team mates are nearly all killed in an explosion. Given that Gaara used to be an Ax-Crazy psychopath who regularly threatened to kill Kankuro, this is quite a drastic change. Sadly, it all goes horribly wrong when poor Kankuro gets his arse handed to him by Sasori, and Sakura barely manages to save him.
There's also the Raikage. When he is told his brother Killer Bee was killed by Akatsuki his response was to call an international meeting to stop Akatsuki. When he found Sasuke he became so hellbent on revenge that he didn't even care if he lost his arm to kill him. And his leg, too, if Gaara hadn't stopped him. It's also possible that, despite all that, if Gaara hadn't stopped him, Sasuke would have died.
Possibly one of the most well-known examples is Itachi Uchiha towards Sasuke and the Leaf Village. The whole Uchiha Massacre episode was set up to protect Konoha from the coup d'etat his own father was planning, and though he was able to kill both of his parents, his best friend, and his girlfriend, he couldn't bring himself to kill Sasuke. He even went so far as to threaten to leak the village's information to every rival village if Sasuke was ever hurt due to something the leaders of Konoha had done or ordered. Let's not even discuss the deal with their fight to the death or the fact that Itachi let Naruto get away from him after said Idiot Hero promised to straighten the then-rogue Sasuke out. And after what happened to Madara when Sasuke saw his Sharingan... let's just say it's rather frightening to imagine the exact consequences of someone doing anything to permanently injure Sasuke.
Neji Hyuga becomes this for Hinata in Rock Lee's Springtime of Youth, where he beats up and kicks everyone who wrongs Hinata. He mostly does it to Naruto, because Naruto obliviously says inappropriate things to Hinata, which leads to Neji beating the crap out of Naruto. There is also this instance where Hinata timidly approaches Naruto, Neji and Lee. Neji tells Naruto and Lee to be patient with her and wait for her to speak. More than twenty minutes pass and then Hinata finally gathers the courage to say something about the weather, which makes Naruto and Lee burst out That was it?!, making Hinata feel bad and depressed, which makes Neji kick Naruto and Lee into space.
However, it can be played straight with the older brother Ace. When he found out that Luffy was being targeted, he wasn'tpleased at all.
Mad Scientist Komui Lee of D.Gray-Man is very protective of his younger sister, Action Girl Lenalee. So much so that Link, wanting to go into a room which Allen had locked him out of, threatened him with, "you leave me no choice. I am forced to tell Supervisor Komui that you are in a sealed room with Lenalee Lee," to which Allen quickly apologized and let him in. There is also an anime-only filler episode where Komui mistakenly thinks that Lenalee is going on a date with one of the Order's scientists and has to be convinced to simply stalk them all over town instead of outright killing the guy. He then repeatedly tries to attack and is comically restrained, until eventually he manages to summon a huge robot to aid him.
This trope is revealed to be Folken Lacour de Fanel's main motivation in The Vision of Escaflowne. Having failed to pass the Dragon rite (and having been mutilated in the process), Folken joined Dornkirk in hopes that he could create a world where his younger brother Van won't have to fight as king.
In the manga of Death NoteVillain Protagonist Light Yagami claims in an early Motive Rant that part of the reason he's killing criminals en masse is to make the world a safer place for his little sister Sayu.
It might even be somewhat true. Sayu is, after all, the only person he refused to kill when it would be to his advantage to kill her. Then again, it could be that he just realized Mello would keep kidnapping people until he finally did something about it other than kill their prisoners.
Deconstructed in Overman King Gainer: Asuham Boone will react violently (and disproportionately) towards any slight to his younger sister Karin, even minor or imagined ones. The whole reason why he wants to capture Gain and thwart the Exodus is that he had convinced himself that Gain had seduced and abandoned her, while the truth is that Gain and Karin had a one-night stand and Asuham overreacted. It gets so bad that during the last episodes, Asuham goes insane with rage and decides to destroy both Gain and the Exodists by awakening the Overdevil, a world-ending Eldritch Abomination. He gets better, though... Perhaps too much.
When Last Order is kidnapped in A Certain Magical Index, Accelerator reverts back into the person he used to be, except in an effort to save her. Slaughter ensues. The lesson in all this? Don't mess with Last Order if you value your life.
Misaka has shades of this herself when she discovers what's happening to her sisters/clones and what she's willing to go through to stop the Level 6 Project.
Kunihiko and Akihiko Ichinose become this, trying to protect their sister Kaoru from a Smug Snake aunt who wants to become her tutor and steal the fortune she inherited from their Missing Mom alongside a Phony Psychic accomplice. They kill the psychic first, and then they murder the aunt when she finds out and tries blackmailing them. Only the psychic was their mom under a disguise, having faked her death to save her business and watch over her kids. Whoops.
Both inverted and played straight in Fullmetal Alchemist. Edward is multiple times in the series confronted with the question of how far he's willing to go to save Alphonse, but sanity and morality usually win out. However, in an early episode of the 2003 anime adaptation, Alphonse comments that if something were to happen to Ed, Al might well go on a killer rampage just like Scar did.
Scar counted as one in the manga and second anime himself (In the first anime they had a rockier relationship). The realization of his brother's death sent him into an uncontrollable rage, during which time he killed the Rockbells. He later moves on to hunting down and murdering State Alchemists to avenge him.
Phoenix Ikki of Saint Seiya embodies this trope when his little brother Andromeda Shun is involved, to the point that putting Shun in life-threatening danger became the default way to work Ikki into any given storyline.
In Chrono Crusade, when Rosette Christopher's little brother is kidnapped by the Big Bad, she makes a Deal with the Devil and spends four years training as a Church Militant. The moment she discovers the location of her brother, she travels from New York to San Francisco, guns blazing, killing any demon that attempts to get in her way, all while being the only Badass Normal in the manga (the anime eventually gave her holy powers). In the manga, it all leads to a battle between her and Joshua Christopher himself who is strong enough to tear apart a massive army of demons with his bare hands. And she kicks his ass.
There's also Fiore, who considers herself as something of a big sister to Joshua. She seems gentle and demure at first, but when the time comes she doesn't hesitate to die in an attempt to protect Joshua, even going directly against her master's orders in the manga.
Durarara!!'s Shizuo Heiwajima tends to act like this in regards to his little brother, Kasuka. While he has reason to be concerned for his brother's safety and privacy (Kasuka's a popular actor, and has been stalked by both rabid fans and the paparazzi in the past), Shizuo takes it a bit too far by immediately becoming suspicious of any mention of his brother by strangers, innocent or otherwise. Said suspicion usually escalates intotree-throwing.
Touya from Cardcaptor Sakura has some shades of this because he promised his mother on her deathbed that he'd protect Sakura.
Some? He jumped over a fence and tried to beat up a 10 year old boy for bullying Sakura! Admittedly, he probably sensed that the boy was a powerful magician and knew right away that he would someday take his sister from him, but he spent the rest of the series disliking and competing with Li. Not to mention the reason why he is always at a different job...
Inverted in Rurouni Kenshin in that Enishi is a Knight Templar Little Brother. So concerned about his older sister's well-being and so seething with pure hatred toward his brother-in-law for killing Tomoe's fiance, though unbeknown to Kenshin at the time that Enishi collaborated with a league of Shogun assassins in order to kill Kenshin - and this was while he was ten or twelve. After witnessing Kenshin killing Tomoe (though by accident) Enishi became a little psychopath before getting involved in organized crime in order to one day take revenge on the man who killed his sister.
Lets not forget Urd from Ah! My Goddess! Urd makes it clear that she will do anything and use any power she has to in order to protect her sisters, Belldandy and Skuld. Urd has to continually "punish" Aoshima after his latest rape attempt against Belldandy or Skuld (he tried to force himself on her when Skuld grew up after Yggdrasil crashed). Urd stood up to her own mother Hild, the Daimakaicho, telling her to stop tormenting "her little sister" Skuld after Skuld incurred her wrath. Also, Urd actually passed the test to become a First-Class Goddess (the highest level that every goddess tries to attain) but declines because First-Class Goddesses have to use their powers to protect everyone; Urd only wants to use her powers to protect her sisters. Since Belldandy goes absolutely postal if anyone even blinks wrong at Keiichi, it is easy to overlook that Urd is a Templar Knight Big Sister as well.
More than one Sympathetic Murderer in Detective Conan is a brother or a sister seeking revenge for the death or ruin of their younger or older sibling. Some examples are Toshiya Tadakoro (kills his boss for causing the death of his Ill Girl sister while Moving the Goalposts), Akiko Sayama (kills a corrupt PC programmer for causing her older brother's ruin and suicide), Chieri Aki aka Eri Akechi (kills two Alpha Bitches for framing her long-lost younger sister for drug trade and making her commit suicide), Masao Kouda (doesn't kill anyone but becomes a bank robber to hide his loot in the backyard of the man who killed his younger sister and force them dig in to find the girl's corpse), Midori Nozaki (brews a mix of Fatal Method Acting and Thanatos Gambit to both commit suicide and take the woman who made her sister kill herself down with her) and others.
Depending on your interpretation of their relationship, Spain is either this, Papa Wolf or somethingelse entirely to Romano.
In Hunter × Hunter, Illumi Zoldyck's feelings for his younger brother Killua are...intense. He is extremely protective and possessive of him. Being creepy even by the standards of a family of assassins, his protectiveness manifests in twisted ways. Such as shoving a needle into Killua's brain to make him into a coward who will flee from any confrontation against a powerful opponent. Threatening Killua is also Illumi's Berserk Button. During an otherwise civil conversation, the very moment Hisoka implies he wants to kill Killua, Illumi puts on a Nightmare Face and radiates a killing aura so intense that Killua can feel it from miles away. He immediately calms down when he realizes Hisoka was trolling him. It should be noted that Illumi only feels this way towards Killua. He seems indifferent towards Milluki and Kalluto and he wants to kill/enslave Alluka.
Kagetsu of Shamanic Princess adores his little sister Sara. So much so, that when the Powers That Be give her as an offering to a powerful sentient magical artifact, he steals the artifact at the risk of causing the collapse of his world to save her.
During the years of Pretty Cure, many of the Cures who have siblings got angry when a villain dared to attack those said siblings, but the only one who got really mad was none other than Cure Black, who actually killed a villain (Gekidrago) for revenge of hurting her Annoying Younger Sibling.
Shinnosuke from Apocalypse Alice, who tries very hard to be The Unfettered in wanting to save his little sister. To achieve that goal, he's sworn off anything unnecessarily detrimental to him, such as sticking his neck out for others as he can't afford to die before rescuing his sister. That said, keyword is tries.
Katina Choovanski is quite capable of taking care of herself, but her crimelord sister still keeps interfering in her life - sometimes by killing people horribly because they tried to hurt Katchoo, sometimes by trying to ensure that she ends up with the "right" love interest. Our heroine does not appreciate this.
Raphael, while not being the most even-tempered character, is psychotically protective of little brother Michelangelo - to the point where he states that the 'thought of Mikey being hurt makes him experience violent rage' (see The Other Wiki). And considering how violent Raphael is usually...
She-Hulk and the Incredible Hulk are cousins, but they have a Brother/Sister relationship. She-Hulk can take care of herself most of the time, but pity the poor fool who harms She-Hulk within proximity of The Hulk. She-Hulk herself can be extremely overprotective of The Hulk as well. Iron Man got his ass handed to him by Ms. Hulk when she found out Stark exiled Hulk into space.
How about that time Nightwing freakin' killedThe Joker with his bare hands when he thought he killed Robin? Of course, ol' Bats quickly revived him afterwards, but even still.
Thor's brother Loki is a manipulative, deceitful god of lies and mischief, and he often does bad things and sometimes gets punished for them… but if you dare hurt him unjustly Thor will smite you from the face of the Earth.
In The Uncanny X-Men, Colossus was this toward his sister Illyana, to the point where he allowed himself to be possessed by the demon Cytorrak and became the new Juggernaut to protect her, losing his humanity and the love of Kitty Pryde in the bargain. Cruelly subverted; Illyana herself had manipulated Colossus into becoming the new Juggernaut to demonstrate that the sweet innocent girl Piotr called "Little Snowflake" was long-gone, replaced by a happily psychotic monster who was not worth saving. When he learned the truth, Colossus vowed that he would kill Illyana if they ever met again.
The Kage Tenshi's The Red Monarch's Sousuke Miyabe is one for his little sibling Tatsuki. To put things into perspective, 10 years ago, he kills several Center agents, nearly kills M.O.M and Colin Price and even kills his own parents when they try to protect Colin. Tatsuki saw ALL OF THIS, which comes back in literal nightmares. To get the full story, if you read the spoilers, read The Red Monarch until you reach the most recent chapter, as it is still ongoing. The link directly to the story is provided for convenience.
In Murderer's Row, a Prison AU of Red vs. Blue, Grif and Simmons were thrown in prison for murdering a man who hospitalized Sister.
In Kitsune no Ken: Fist of the Fox, Neji is a Knight Templar Older Cousin for Hinata, going to varying lengths to keep Naruto away from her, up to and including attempting to manipulate events so Naruto will get kicked out of Konoha High School. Hiashi approves, especially after they find out Naruto was one of the legendary Nine Terrors, but Hinata herself is not pleased.
Do not hurt Molly in Sleepaway Camp II: Unhappy Campers, emotionally or physically. Angela will kill you for it. Just ask Ally, who was DROWNED IN AN OUTHOUSE for committing that crime!
When the protagonist of Sick Girl finds out her little brother's being bullied, she hunts the three bullies down, kills one of them, and forces the lead bully to drown the other, then ties him up in her barn and spends several days torturing him to death.
Dead Mans Shoes. Somewhat after the fact, older brother Richard returns from the army after discovering his brother has been bullied and goes about giving the bullies a taste of their own medicine with interest.
Haru from Beverly Hills Ninja: "I may not be the Great White Ninja, I may not be one with the universe but I know this...NO ONE MESSES WITH MY BROTHER!"
The titular character of Ginger Snaps starts out as a Knight Templar Big Sister and gets steadily worse over the course of the movie, as her transformation into a werewolf makes her more and more aggressive. For example, she kills the janitor because she thought she saw him look down Brigitte's top earlier in the movie.
In the 1938 Yiddish film Mamele, Khavkhe (Molly Picon) takes care of her whole family. A gangster, Max Katz, recruits her brother into a burglary gang, and courts her sister (with the intention of turning her into a prostitute). Khavkhe manages to pickpocket Katz's gun, and makes him write a letter breaking up with the sister. ("You may be a Katz (cat), but you'll leave here as a kitten.")
Tony Montana from Scarface (1983). He's just a tad bit overprotective of his little sister Gina, even going as far as murdering Manny Ribera, his best buddy for sleeping with her right behind his back. Though unbeknownst to him, said friend is already married to Gina the day before. Worth noting that more than one critic has observed an incestuous undertone to some of Tony's actions which Gina also accuses him of when she tries to kill him in revenge.
Alpha Dog. When Jake Mazursky learns that Johnny Truelove has kidnapped his little brother Zack to get back at Jake, he promises his estranged stepmom that he will do anything to get Zack back. Jake immediately goes on a rampage, crashes a local house party held by some of Truelove's friends by beating up half the people in the room and when Truelove calls him up, Jake threatens Truelove with enacting brutal and painful vengeance unless he releases Zack. Subverted however, as it doesn't help Zack's return at all and prompts Truelove to relocate with his victim.
American History X: Derek threatens to feed Cameron's heart to him if he continues trying to corrupt Danny like he did with him.
Niko Leandros would burn the world to protect his brother. The only reason Cal ever gets near to any combat is because Nico is a firm believer that the only way to be ready for combat is to actually participate in it.
Cal reciprocates the sentiment just as strongly. And considering he's slowly slipping into a Sociopathic Type VAnti-Hero over the course of the series with Niko as his sole Morality Chain...the question isn't if he kills you, but how much he'll make it hurt before he does so.
James Vane from The Picture of Dorian Gray. His little sister Sybill killed herself when he was working abroad, so when he found out, he swore to kill the culprit, Dorian Gray. He almost got his wish, but was accidentally shot to death.
In Hell's Children, by Andrew Boland, you never, ever, F%*& with Acheri, or her defacto big Brother, Precision, will Mind Rape you.
If you decide to kill and eatHannibal Lecter's baby sister, don't be surprised when her brother tracks you down and makes a shish-kebab of your cheeks. With mushrooms.
Gretchen Richter in 1632 is a gender-switched version of this, for her brother Hans.
Max in Maximum Ride. She's normally a calm and easy-going person. However, she treats the flock like her own brothers and sisters. If you're foolish enough to threaten one of them, you're gonna regret it.
Thresh in The Hunger Games seemed to take Rue, the girl from his district, in as a sister because when she died, he was infuriated by her death. During the feast, Thresh overheard Clove talking about Rue to Katniss. Thresh grabbed Clove off Katniss, strangled her while holding her one foot off the ground, and yelled at her about Rue. He then kills Clove to avenge Rue.
You do NOT mess with Thomas Raith's siblings, not even if you're their father. Especially his younger siblings Inari and Harry (who, though younger, reciprocates the sentiment as Thomas is his only living relative left and shares many similarities with him).
Live Action TV
Scully's big brother Bill. It's really annoying, since this is a woman who has been shown to be able to handle herself time and time again amongst a predominantly male-oriented workplace. She goes out of her way to never be accused of special treatment because she's a woman. And yet she lets Bill push Mulder around all the time.
It's not all the time. Bill and Mulder meet about twice in the entire course of the series, and though Bill bad-mouths Mulder, the fights between the men usually happen when she's not around. Not to mention Mulder is capable of holding his own in those fights. The annoyance of this trope comes more from the fact that Bill is making assumptions with half the story.
Also slightly justified when it comes out that Bill has always acted like this as a result of having to be "the man of the house" when his father was away at sea, and that Dana has always taken his opinion with a grain of salt. It's not only Mulder Bill has a problem with, but her having a life different from what was expected of her: joining the FBI rather than practicing medicine, not getting married, having a child out of wedlock...Mulder is only a part of it, though a substantial part.
And though his little sister is gone for the entire series, Mulder could most definitely count. He's spent his entire adult life searching for and avenging his sister's kidnapping. Samantha Mulder is the reason Mulder works on the X-Files and why he is so determined to uncover the conspiracy. He does not hesitate to hurt or kill someone in the process of finding her.
If you think of even putting a single finger on Sammy, well then you must be fist and gun proof. Or if you're an angel, messing with Sam to get Dean's agreement is a bad idea.
After making Sam watch Dean die over a hundred times in one episode to desensitize Sam to losing Dean (it doesn't work; after Lilith kills Dean, Sam turns to The Dark Side for vengeance and the power to save the world) the trickster/Gabriel comments that:
"Whoever said Dean was the dysfunctional one has never seen you with a sharp object in your hands."
You probably don't want to mess with Castiel on Dean's watch (or vice-versa) either, although they are more like unofficially adopted siblings
Lucifer and Michael might have been trying to destroy one another, but Lucifer got pretty vengeful when Castiel molotov'ed Michael.
Firefly: Just don't mess with Simon's little sister.
No matter how he tries to negate it, ''John Casey'' seems to take that road in regards to Chuck, even though he always claims it's to protect the Intersect. The Casey Doth Protest Too Much, indeed.
Of course Casey's just protecting the Intersect. That's why he had his super kickass special forces team abort their current mission and reroute to Ellie's wedding where they crashed through the roof and neutralize all the baddies in a Big Damn Heroes moment, just because Chuck was in trouble. Which happened in the brief interim when Chuck wasn't the Intersect.
It is revealed in one episode of Malcolm in the Middle that Reese is this to Malcolm, and that was the only thing keeping him from getting beat up for his smart mouth. Malcolm finds this out when his brother decides to be nice for the rest of his life.
Don Eppes of NUMB3RS. How many people were expecting him to kill someone during Breaking Point?
Justin, from Wizards of Waverly Place, tells his younger sister, Alex, that he always thought he would intimidate her boyfriends if they got out of line and in Wizards vs. Werewolves he fights Mason (who is in full werewolf mode) for her, with his bare hands.
Don't get in between Iris Crowe's little brother and... anything. Just don't.
Burn Notice: Do not pick on Nate, Michael Westen's little brother. It is unhealthy. As those involved with Nate's assassination in season 6 will discover.
In the episode "Comrades", there is a deconstruction. Michael meets a Russian woman, named Katya, whose little sister Elena is kidnapped by the Russian Mafia. Michael has to convince her that being a vengeful big sister might actually not be in the little sister's interest.
Prison Break: Don't mess with Linc or Michael will come up with an elaborate plan to destroy you. Don't mess with Michael or Linc will kick your ass.
In Season 2, Willow is kidnapped by a group of vampires looking to revive The Master, intending to use her (and a few others) as a human sacrifice. Xander, who blames this on Buffy being depressed and distracted after being clinically dead, threatens to "kill [her] myself" if Willow isn't rescued unharmed.
Jing Tian from Chinese Paladin 3 boomerangs from having a simple Big Brother Instinct to full on Knight Templarhood. For example, his reaction on seeing his best friend "threatening" his little sister with a Healing Shiv is to attack and nearly kill him.
Cesare Borgia is this to a somewhat frightening degree in regards to his younger sister, Lucrezia. By the third episode he's threatening to cut her husband-to-be's heart out with a dinner knife if he's "ungallant". He shows some signs of this towards one of his younger brothers, Gioffre. Not so much with Juan.
In Arrow, Oliver's sister Thea takes a new drug called Vertigo and drives under the influence due to family issues. So Oliver does the reasonable thing and visits her in the hospital, and then he tries to support her legally, especially after the judge wants to make an example of her, and tries to help resolve the family issue that led to the car crash. So why is this entry here? Because he also tracks down the drug lord that made the drug and sells it, known as The Count, and injects him with the very drug in its purest form in an insanely large quantity, leaving him insane due to the drug making him imagine being in the most excruciating pain possible. The episode ends with him spasming violently and screaming strapped to a hospital stretcher as a doctor explains his extreme condition. And he never got NEAR Thea, only being an indirect cause!
Friends - Ross attempts to be this to Monica after she got engaged to Chandler, stating that if Chandler ever hurt her, he was going to "...hunt him down, and KICK HIS ASS!". The rest of the friends (including Chandler himself) found this hysterical.
On Youngers, Jodie's brother is this. He threatens to beat Jay up for just looking at her and when Jay breaks up with her, she tells him, which leads to a beatdown.
Myths and Religion
Absalom from The Bible, who learns that his sister Tamar was raped and humiliated by their half-brother Amnon and, instead of telling their father David to demand punishment, he goes and kills Amnon himself.
In all fairness, David was already well aware of what had occurred (see 2 Samuel: 22-23). His response was to be 'very angry'... and do absolutely nothing. Absalom decided to take justice into his own hands - it was obvious none was forthcoming from their father, the king.
In a similar incident two of the sons of Jacob killed everybody in a village for mistreating their sister. Presumably they wished the feud to end right away.
To put it into perspective, Jacob's daughter Dinah had been raped by a prominent member of the village who fell in love with her. Despite the circumstances, he and his father wanted to smooth over relations with Jacob's family and offered to pay for her dowry as if she'd still been a virgin, which was actually quite a generous offer at the time. Jacob seemed to warm up to the offer, but his sons demanded that all the men of the village circumcise themselves to become part of Jacob's tribe. The men agree, with the idea that they will now be able to intermarry with Jacob's tribe. After all the men are circumcised and helpless, though, Dinah's brothers take advantage of the situation and slaughtered the entire village, taking only women and children alive as slaves. When Jacob bemoaned them for the actions, saying that no one would trust him now, they only reply: "Our sister should not be treated like a whore." Jacob went along with this right up until just before he died, at which point he called them in and disinherited them for wiping out an entire village for the actions of one man. (Historians believe this story was written in part to explain why those tribes of Judea were on the bottom of the social ladder.)
In The Mabinogion, Efnisien gets very angry when his half-sister, Branwen, is married to the king of Ireland:
And thereupon he rushed under the horses and cut off their lips at the teeth, and their ears close to their heads, and their tails close to their backs, and wherever he could clutch their eyelids, he cut them to the very bone, and he disfigured the horses and rendered them useless. And this is just his initial reaction.
And when Branwen's husband starts treating her like a slave, all her brothers go to war to defend her honour.
Apollo of Greek Mythology tricked his sister Artemis into killing her almost-love-interest Orion because he didn't want her to break her vows of chastity.
The filecards for G.I. Joe members Barrel Roll and Bombstrike, and the filecard for Cobra operative Blackout contain a story where Barrel Roll acts as Knight Templar to rescue his sister Bombstrike after blacksheep brother Blackout kidnaps her.
Gulcasa of Yggdra Union. Serious harm befalling his younger half-sister Emilia induces Berserk Button-level rage, and towards the end of the game—directly after he learns that she has been killed by the Royal Army—said rage causes him to reject Yggdra's repeated pleas for a cease-fire to the war that has effectively destroyed most of his country. Usually, he's a shrewd and reasonable ruler, but in this case...
Don't get anywhere near The Legend of Zelda: The Wind Waker 's Aryll, or prepared to be mercilessly hacked into pieces by the Master Sword. Good thing Link's an only child in all of the other games.
Zelos Wilder in Tales of Symphonia. Though Seles seems to hate him for being born since her mother tried to kill him so that Seles could be The Chosen One instead, but Zelos' mother saved him, and as a result Seles' own mother was executed, he will "throw anyone who so much as looks at her wrong down the Palmacosta harbor". Whoa, dude, chill.
Guy Cecil from Tales of the Abyss — his older sister Mary may be dead, but he practically raised Luke. Guy's a pretty chill person normally, but if you so much as threaten one hair on Luke's head, you press his Berserk Button. Especially evident during the Summit, when Jade suggests Luke die in Asch's place, and Guy grabs Jade by his collar, actually yells, and calls Jade a bastard for it.
In Tales of Xillia 2, this ends up being inverted if you choose to see the bad ending by making Ludger refuse to kill his older brother Julius (Something the latter wants, as he's doomed, and it's the only way to extract a Soul Bridge from him, a key that the party needs to save the world). Gaius proceeds to state that he'll do the deed himself, then things get ugly from there. Really ugly.
Targeting Axel's little brother is not a good idea, something a repo man learned the hard way at the end of the Axel story. And as aforementioned Dark Hero learned the hard way, do not abduct Adell's siblings or Rozalin's "servants". For any reason at all.
The plot of Disgaea Dimension 2 can be summed up as this given that Xenolith would allow the Netherworld to transform into something similar to Celestia as well as give up his life if it means saving his little sister Etna. Alas Etna constantly refuses to acknowledge that Xenolith is her older brother even with her life on the line. Of course one particular bad ending has Xenolith fail to save Etna when she refuses to acknowledge that Xenolith is her older brother and destroys the artifact that is destroying the Netherworld with her powers in it. That power ends up killing her and everything he has done had been for nothing.
And let us not forget that Mega Man and Proto Man are basically teaming up together in 10 to get the vaccines for Roboenza, to cure Roll. Hell Mega Man goes after Wily before he goes after the Robot Masters. Whoever created this virus better have good life insurance. Yeah, Wily's life insurance is up to date. Bastard's stillalive.
Blues in Mega Man Megamix fits the 'morally questionable actions' part of the trope (what separates it from Big Brother Instinct), up to and including threatening to kill Kalinka Cossack if anyone gets involved in the Mirror Match between Mega Man and Copy-Mega Man. Of course, not only was there the risk that they'd shoot the wrong one if they'd tried to help, but Dr. Wily had given Shadowman orders to take advantage of the chaos to kidnap Kalinka. Blues couldn't help Mega Man fight without letting Kalinka get snatched, and he needed some way to keep the other robot masters present from trying to help Rock and causing the Fog of War Wily was going for. If he hadn't threatened her, they would have tried to go through him, and Shadowman would have taken her while he was distracted. By the side of the one with the shield was the safest place for a human among a lot of robot masters with itchy trigger fingers, and he did succeed in saving everyone except Copy-Mega Man. There are plenty of other examples.
Consider that Wily based his own robot designs on Proto Man's specs. Bass would then be multiple inversions of the trope: he's on a constant mission to destroy Mega Man, and if Wily pisses him off he'll gladly take it out on the other Wily bots...who are also Proto Man ripoffs.
Any attempt to harm Mega Man X or Zero in the other's presence will result in the survivor of the pair bringing the Big Bad's fortress down on top of you. In later games, if you take out Axl, you now have to deal with TWO pissed-off Knight Templar Big Brothers.
Prototype: Whatever happens, whoever you are, whatever the reason, never nevernever harm Dana Mercer in any way, because the consequences for doing so come in the form of her big brother, Alex. Alex is a shapeshifting, superstrong, superfast, superagile, incredibly resilient Humanoid Abomination with issues, and Dana is the one thing in the world he cares about. Alex once prevented Dana's capture by punching through the torso of a soldier wearing body armour. Alex ate a few dozen people who happened to be vaguely connected to Dana's location when kidnapped, and two monsters, and the mastermind herself. Alex is alreadynot wholly stable. Try something safer, like playing Chicken with the sun. And in a wholly depressing twist, Alex isn't even really Dana's brother, simply a virus wearing her brother's corpse like a suit. The real man cared so little about his younger sister that he knowingly condemned her to a hideous death with the rest of the world when he released the virus, before he died and it consumed his remains. This was a man so devoid of love that a flesh-eating virus designed specifically to wipe out humanity cares for his sister more than he did. What a dick.
In the sequel, Alex appears to have dropped this trait, alongside almost any positive traits he had. Though it should be noted that despite Dana working against his evil plan, he chooses to capture her, rather than just kill her like he did Heller's other contact, Father Guerra.
In Dragon Age II, the player character can act like this towards a younger sibling, Bethany or Carver. And considering what can potentially happen to them throughout the game, it's no wonder.
It's much more prominent with Bethany however, such as when Ser Wesley identifies Bethany as being an Apostate, causing Hawke to immediately step in front of her and stare down the Templar with a Death Glare. This even happens if Hawke is a Mage, putting themselves potentially in the line of fire to protect her. Should Bethany go to the Circle, Hawke similarly threatens to disembowel any Templar who touches her, only relenting when Bethany explains she's going peacefully and by her own accord.
A possible sidequest in The Godfather 2 involves you being told to beat up or kill someone taking advantage of the quest-giver's sibling.
In Borderlands 2, it's implied that Scooter acts like the trope towards his sister, Ellie, despite Ellie telling Scooter to leave her alone and stop setting her customers on fire.
Phoenix Wright is like this with Maya Fey, his assistant. Since this can sometimes be thought to be a Ship Tease by some fans, it is noticeable that he would run across a burning bridge to save her from a cave, and break down a door that led to her after hearing a gunshot in the room. And Maya is also this way towards her cousin, Pearl, as she was enraged when Phoenix suspected her of any wrongdoings in the second game.
Another example is the Blackquills from Dual Destinies. Simon is this towards Athena, as he went through death row for her sake and was willing to threaten Phoenix in court if he tried to prove her involvement in a case. Same with his older sister Aura, who held an entire building of people hostage to allow her brother to be acquitted from false murder charges. Guess this runs in the family.
Clover in Nine Hours, Nine Persons, Nine Doors has a suspicion on who killed her older brother, Snake, in Door 3. In the Axe Ending, when she calculates who the murderers are, she acts on said misconception, killing both Santa and Seven, the suspected murderers, and June, the protagonist's lover for defending them, before turning against the protagonist himself. Though depending on how well the game is played, the players can get the True Ending, where Junpei befriends her and inadvertently stumbles across a crucial clue that indicates her brother is still alive.
Kaoru "Sou-Banchou" Ichimonji from Tokimeki Memorial 2 is revealed to be this if you romance his younger sister, Akane. And this is the main character's own fault: Sou-Banchou witnessed first-hand how utterly heartbrokenthe poor girl was was when you-as-the-MC moved outta the city, so he swore to pretty much punch the HELL outta any guy who made her cry.
Johan of Anders Loves Maria has long been violently protective of his little sister, Maria. Leads to him committing acts of vandalism and assault in her name.
Mijuu of Juathuur is Dejoru's overprotective big sister.
Panthera has Jason, who threatens his best friend Fletcher (who finally got a date with Taylor) as such:
Jason: If you do ANYTHING to her, in any way, shape or form, I will quite literally #%$&!$# EAT YOU. And you have NO idea just how DEEP I could BURY what'd be left of your sorry carcass. UNDERSTOOD?
Lenny Priestly of Survival of the Fittest v3 will do anything to make sure his twin sister Elizabeth is safe. Literally anything, as he kills people for this reason. Though possibly a subversion, as it has been implied that he used her as an excuse to kill, especially since he went Ax-Crazy and stopped being an Anti-Villain.
In Brennus, Amy is very very protective of her younger brother Basil, although she doesn't tell him about everything she does. He still thinks some of the Hemogoblins got away. She tore them limb from limb.
Dee Dee from Dexter's Laboratory may appear to be a flake, but you do NOT harm her little brother. Or she will kick your ass.
Kenny McCormick from South Park displays this in the episode "The Poor Kid". If you have the gall to mess with Karen McCormick, Kenny proves that he Would Hit a Girl quite viciously, as Jessica Pinkerton finds out. The rest of the school gets the hint that the girl is off limits.