Tom does everything he can, which isn't much, to keep his brother from being infested and goes nuts at the thought of the Yeerks getting to Jake. Jake displays a similar protectiveness towards Tom besides being younger. Also, Elfangor towards Ax.
At one point in the series, when Jake and Controller Tom are talking (which they do often, being brothers, obviously), "Tom" lies to Jake to cover his status as a Controller, and, if only for a second, Jake notices Tom's face twitch strangely. Jake suspects that Tom is trying so hard to fight against his Yeerk that he momentarily wrestled control of his body away from the Yeerk, which is established as impossible.
The entire storyline of Along The Winding Road is because of this. Despite not even knowing if her little brother is alive, Charlotte is determined to cross the zombie-infested remains of Texas just in case he needs looking after.
In The Blue Codex both Trini and Jazlyn are very protective of each other even going so far as calling each other sisters even though technical they are step-sisters.
Farrah in The Gauntlet 2017 goes into the game to find her little brother Ahmad after he rushes in.
Carl Dalisay to Noel Dalisay in Mars Evacuees, by Sophia McDougall. Not only does he physically protect his younger brother, Carl goes to great lengths to make sure Noel doesn't realize how dire things are.
Violet Baudelaire in A Series of Unfortunate Events frequently recalls the promise she made to her parents (twice, the first time being when she was two) to protect Klaus and Sunny. Given what the three children survive, she seems to do a pretty good job.
Played with in Dark Places. In the Satanic Panic of The '80s, Libby Day testified against her older brother, Ben, for the murders of their mother and two other sisters. In the present day, she realizes over the first few chapters that it might not be that simple. When the Plot Twist towards the end of the book happens, it's revealed that Ben didn't kill his family, but he did stand and watch his girlfriend, Diondra, strangle his sister Michelle after Michelle found out that Diondra was pregnant. Immediately after Michelle dies, Ben runs to find Libby and tells her to stay where she is so Diondra won't kill her, too.
Harry becomes this to Molly when her magical talents manifest. It becomes quickly obvious that both are suppressing sexual feelings for each other, and Molly breaks first. Harry makes sure to keep their relationship as platonic as possible.
He does it to his half-brother Thomas despite the fact that Thomas is the older brother.
Thomas also has a brother instinct. He has followed Harry around town without Harry's knowledge, and in several books the only time Thomas hasn't come to Harry's aid was when the White Council was directly involved. He is also very protective of his little sister, Inari.
The Chronicles of Narnia: Peter is this toward all three of his siblings who he is very protective of, but is especially this for his youngest sister Lucy. He vehemently rebukes Edmund for humiliating her in the beginning of the series, up to the point where he looks like he's about to deck his younger brother in the movie adaptation. Later, despite having never wielded a sword before, Peter kills the monstrous wolf Maugrim to protect his other sister Susan.
The movie version of Prince Caspian portrays Susan falling in love with Caspian. Peter definitely seems to oppose Caspian in every matter, getting to the point where they almost battle each other. It's almost as if Peter is destined, from the beginning, to hate the guy and bash him, while his little sister is instantly swept off her feet by him.
Fred and George seem to take a slightly more protective role when it comes to Ginny. In the first book, they tell her not to cry and promise to send her "loads of owls" in the first book. They also tell her they'll send her a Hogwarts toilet seat, which makes her laugh. They used to pick on Ron at school and at home, but they love him regardless of their treatment towards him and don't hesitate in attacking Malfoy when he insults their younger brother in the fifth book. Later, Fred and George are seen tag-team comforting a first-year who suffered a punishment from Umbridge. They also take Harry under their wing, sticking up for him when he's being called a liar about Voldemort's return and cheering him up when everyone else believes he's the Heir of Slytherin. They also get revenge for Harry by tricking Harry's bully of a cousin Dudley into eating a Ton-Tongue Toffee.
Mr. Weasley: It isn't funny! That sort of behavior seriously undermines wizard-Muggle relations! I spend half my life campaigning against the mistreatment of Muggles, and my own sons— Fred: We didn't give it to him because he's a Muggle! George: No, we gave it to him because he's a great bullying git. Isn't he, Harry?
In the movie version of Goblet of Fire Arthur specifically leaves them in charge of Ginny while Death Eaters are running around. Given Arthur's Papa Wolf status, that says a lot. In the books, Ginny stays with the twins as well.
Ron Weasley reveals this in Harry Potter and the Chamber of Secrets while nearly cursing Malfoy (with a broken wand that has already backfired once on him) for being mean to Ginny. Two sure-fire ways to anger Ron are to insult either Ginny or Hermione.
Ron gets a bit of this from Percy after the Second Task in Goblet of Fire. Percy, who's supposed to be acting as an impartial judge gives Harry full marks for rescuing Ron and then physically drags Ron out of the lake and onto dry land... to his younger brother annoyance.
Eclipse Hunter: Devon falls under this trope very easily, and fills many different archetypes. He's willing to shoot his own father if it means protecting Daren.
In The Thief Lord, Prosper's protectiveness of Bo is the main motivation behind his character. He runs away from his aunt with Bo to prevent their separation, joins a gang of urchins and steals to make sure his brother is well fed and sheltered, and goes into a major Heroic BSoD when Bo is finally taken away.
The Darkest Powers series gives us Derek. He has a Big Brother Instinct that's Up to Eleven when it comes to his foster brother, Simon. Derek is also a werewolf. So on top of the fact that he is, personality-wise, very protective of Simon (and anyone else he cares about), he also has the wolf's "protect my pack" instinct to deal with. Assaulting one of his "pack" is his Berserk Button. Unfortunately for Derek, since he can't usually control his reaction when his brother is in danger no matter how much he's tried to coach himself, his supercharged Big Brother Instinct has led to at least one My God, What Have I Done? moment.
Len Maynard of Chalet School Practically ever since she is a baby, she is pigeonholed as the 'responsible one' and as the oldest Maynard child, she is often put in charge of the other Maynard children. She has a major one where Margot is concerned, protecting her from Jack's wrath and lying to cover up for her in Triplets of the Chalet School after the bookend incident. Miss Annersley even calls her on it.
Kaladin from The Stormlight Archive develops one of these for every relatively (around 14) young male he comes across in the course of the story. This stems from his feelings of guilt over failing to protect his actual younger brother in the course of a minor border dispute between rival lords of his nation. Every one of these individuals ends up dead.
The Cal Leandros series gives us Niko, who loses his characteristic cool when someone so much as implies that his brother is a monster, and goes berserk with rage when he thinks Cal is dead. Cal is no better, going to extreme lengths to protect his brother.
Although Damon Salvatore of The Vampire Diaries likes to make his brother's life hell, he does go out of his way to save or protect Stefan.
Katniss Everdeen of The Hunger Games will go to any lengths to protect her younger sister Primrose, including volunteering for the titular Deadly Game in her sister's place, a seemingly suicidal gesture.
Katniss takes on this role for Rue as well, doing her best to protect her in the Deadly Game.
Rue to her siblings, and Thresh to Rue. See their character page for how.
To a degree, Finnick Odair is this for Katniss Everdeen, especially in Mockingjay. Katniss is enduring a Trauma Conga Line while she is trying to take care of her little sister and she is a symbol for a rebellion. Her childhood friend Gale isn't around much — doing undercover work with Beetee — and isn't entire supportive while her love-interest Peeta is a captive of the Capitol for the first half of Mockingjay and, as a result of being brainwashed by the Capitol, is no longer himself and a danger to Katinss until he is able to recover toward the end of the book. Katniss's mother, while better than she was in the first book, is unable to truly understand Katniss's ordeal. Finnick, on the other hand, who is himself prone to trauma and currently enduring a seemingly endless case of PTSD, does all he can to make Katniss feel safe and comfortable, despite his own issues. Until he dies. Boggs is this too. Until he dies as well.
Ward of Hurog is very protective of his siblings, which is sorely needed because of his father's abusiveness.
Prince Jonathan and his group of friends in Song of the Lioness developed this for the new page Alan. Even before Ralon started bullying Alanna, they regularly helped and kept an eye out for her. And when Ralon first beat up Alanna, Raoul pulled him aside and roughed him up as a warning. When Ralon later broke Alanna's arm in retaliation, they all thrashed him thoroughly. Despite continuing to harass Alanna, Ralon didn't lay a hand on her again.
And both Alec and Seregil are always and at any time rather protective towards Beka (in Seregil's case it is also a rather paternal feeling; he states himself she's as close for a daughter to him as it will ever get) — including being suspicious about her lover, checking him up, threatening him not to harm her... Beka is not impressed.
Tamir Triad: You better not insult Tobin when his squire/best friend Ki is around. (and in fact thanks to plot this brother-friendship later gets rather difficult)
An even creepier example might be the ghost Brother. Tobin's older twin who died after birth and turned into an angry spirit. After Tobin learned how to keep him in check Brother actually got extremely protective — despite the claims of some characters this troper never could shake the feeling that Brother DOES feel affection and brotherly love towards his sibling. Albeit in a twisted way.
In The Outsiders, everyone was this towards Johnny since his abusive parents could care less about him. Also, Ponyboy's two older brothers Sodapop and Darry, especially after their parents' death.
Brandon Stark was this for his younger sister Lyanna in the series' backstory. When he heard that Crown Prince Rhaegar Targaryen abducted Lyanna, Brandon rode off to King's Landing with a few companions to kill Rhaegar and rescue his sister. It didn't go well...
Jaime Lannister is this for both of his siblings, Cersei and Tyrion. Technically, his sister is a minute or two older than he is, but that's hardly the most complicated thing about their relationship. For the first few books, at least, he'll do anything to protect them — and if it involves killing someone, so much the better. Notably, he is also the only person to show nothing but love for his brother Tyrion, who is subjected to a great deal of prejudice and resentment for being a dwarf — even from their father Tywin and sister Cersei. Tywin and Cersei also hate Tyrion for their mother's Death by Childbirth when she gave birth to him. Jaime never shows any resentment or disgust with Tyrion and is willing to start a war to protect him, and not for political reasons like his father, but because he genuinely loves his little brother.
All the Stark siblings (Robb, Jon, Arya, Bran, Sansa and Rickon) love each other, are (usually) close, and are protective of each other.
Jon, in addition to his Big Brother Instinct for all his half-brothers and sisters, is particularly this for his youngest sister Arya — to the point that his efforts to rescue the girl he thinks is Arya in the latest book lead to his death.
And later, after a rocky start, Jon is this for some of his Night's Watch brothers, particularly his best friend Samwell.
In a gender-reversal, the tough and practical Meera Reed acts as a protector to her much frailer younger brother Jojen.
This does not to all families in this series though: this trope is averted big time in several cases, most dramatically by Gregor Clegane toward Sandor Clegane.
Genna Frey (nee Lannister) mentions that when her weak father agreed to betroth her as a seven year old to Emmon Frey (a second son, and thus a somewhat insulting match for a girl of her rank,) Tywin (himself only 10 years old at the time,) publicly called out their father and Lord Frey for it being a poor match. While she says she did not approve of everything he did as an adult (or even enjoy his company that much) she couldn't help but love him as a protective big brother.
An inverted case for Oberyn Martell with his deceased older sister, Elia. He clearly wants to exact revenge on the Lannisters and wants Ser Gregor Clegane dead for killing her and her children and this ends up getting Oberyn brutally killed. His eldest brother, Doran, seemed to be an aversion at first when he remained passive for not doing anything about his siblings' deaths — much to the contention of his nieces and daughter, butit's straight example when it's revealed that Doran and Oberyn had been planning to destroy the Lannisters for Elia's death all along. Though, since Oberyn managed to kill Gregor at the cost of his life, Doran became bitter that he didn't get the chance to kill Tywin because Tywin's son Tyrion got to him first. However, he's glad that the guy is dead and still continues his revenge plans against the remaining Lannisters. Before that, he's also angry at Rhaegar for putting Elia in a bad light after he abducted Lyanna Stark.
In relation to the Martells, the three oldest of Oberyn's bastard daughters learned from their uncle Doran about Cersei's False Flag Operationto kill their younger cousin, Trystane, en route to King's Landing in order to end the engagement with her daughter by a group claiming to be Tyrion. This revelation made them horrified that the eldest, Obara, wanted give the queen a bag of heads.
In Ender's Game, the reason that Ender was willing to go to Battle School and join the International Fleet was to protect his sister Valentine, although he was actually her little brother.
In the In Death series, Roarke tends to harbor protective instincts toward Eve's female colleagues, though they're not particularly weak. This is probably due to his dead little sister.
In The Voyage of the Jerle Shannara, Quentin Leah already has a pretty serious big brother complex towards his cousin Bek. Following his failure to save a girl he liked, this is magnified a thousand fold, and helps to fuel his change into a badass.
Terry Brooksloves this trope. The vast majority of heroes in the very long runningShannara series have that sort of sibling relationship, starting with the very first book. It reaches its logical conclusion in the final series, The Defenders of Shannara. Its first book, The High Druid's Blade, is basically Big Brother Instinct: The Book.
Bertie of Jeeves and Wooster tends to develop this easily. In one story he suddenly decided that he wanted a daughter for precisely this reason: "Something to look after, if you know what I mean."
Daniar sees her little sister Erowin's safety as her personal responsibility. she doesn't take it well when Erowin is killed.
Rana's makes a big electric BOOM to save Daniar from wind witches. "LEAVE MY SISTER ALONE!"
Averted, oddly enough, with the eldest sister. Zarracka can't stand any of them. In the sequel she develops Aunt Instinct for her nephew, Benji.
In the sequel, Legacy of the Dragokin, Ravage joined Jiahdain's group because his little sister was already part of it and he wanted to keep her safe.
Eldest Whistler in A Brother's Price is this way to her whole family, including sisters-in-law — as she declares, "We Whistlers have an unbreakable rule — you mess with one of us, you mess with us all!" — but particularly her vulnerable brother Jerin. When she believes he was raped by the crown princess, she plans to kill the woman but is enough of a Reasonable Authority Figure to listen to him. Most of Jerin's sisters are like this to some degree. Even Corelle is motivated by the desire to protect him.
Marcus demonstrates this towards Julia in The Mark of the Lion trilogy (at least until her betrayal of Hadassah at the end of book one, after which he declares This Is Unforgivable!), and is the only one who indulges her more headstrong tendencies manifesting from her being Spoiled Sweet.
Jace tends to be very protective of his adopted siblings.
Alec takes his job as the oldest Lightwood sibling very seriously. And he's also the only one of them to be at least eighteen, making him legally an 'adult'.
Simon's sister Rebecca becomes increasingly concerned about Simon's cutting communications with her over the course of City of Lost Souls, and eventually manages to meet with him. When she briefly believes that their mother has brainwashed or harmed him in some way, the way she reacts indicated it would not have gone well for their mom if that had been the case. And when he reveals his vampirism to her, she accepts it almost immediately after getting over the shock, and assures him she'll love him no matter what.
Katsa from Graceling has her moments. She is incredibly protective over Princess Bitterblue. She made it her entire goal to protect the ten year old princess, no matter what it meant for her or anybody else.
One of the sharp contrasts between Huntingdon and Markham in The Tenant of Wildfell Hall is how well Markham handles Arthur. He plays with Arthur, dotes on him without spoiling him, is protective of him, and defers to Helen's parenting choices, all things Huntingdon won't do.
Bush in Lieutenant Hornblower. He quickly picks up on the fact that Hornblower pushes himself far too hard and pulls rank to make sure that he drinks water, eats, and sleeps. He also speaks up for Hornblower's daring plan to take the Spanish fort and insists that he get full credit for it. Later on in their careers, this turns into Bush unconsciously thinking of Hornblower like a son.
Luke and Thalia were this to Annabeth before the trio arrived at Camp Half-Blood. Keep in mind that the two were just 12 years old by then, yet they had to essentially take the role of parents in front of the 7-year-old Annabeth and at the same time protect themselves from monsters who hunted them.
Bianca to Nico, before her death. It's explained that she joined the Hunters of Artemis because for once in her life she wanted to be something besides Nico's big sister. And even after that, she takes a stray figurine for Nico (who was an avid collector), which directly results to her death.
Apollo has this for his sister Artemis in The Titan's Curse (though they don't agree on whether he is actually older than her). He defies Zeus's orders to give Percy information on how to save her.
"Nobody messes with my baby sister. Nobody."
Percy was this to Tyson before he even knew they were brothers.
It's revealed in the first book that the reason Thalia didn't escape home sooner than she would was because she's afraid her mother would endanger her little brother, Jason. When she thought that he was killed (actually presented to Hera), she flipped out and wasted no time to leave her mother. It's apparent that she still has the trauma of losing him as she never told anyone about him.
Hylla to Reyna. She protected her when their father locked them up in his fits of PTSD and accompanied her during their overseas travels after killing their father's insane spirit. And at the end of the second novel, Hylla leads her company of Amazons to assist Camp Jupiter (led by Reyna), which are being attacked by the Giants. Hazel mentions that Hylla is possibly "the best sister anyone could have".
Do not suggest you aren't willing to rescue Nico if Hazel is in the room. She will not like it.
While Nico never really gets a chance to pull a Big Damn Heroes for Hazel's sake, he's still Hazel's big brother. He seems to go out of his way to cheer her on at Camp Jupiter.
Jason develops this for Nico in The House of Hades after he finds out about Nico's crush on Percy.
Reyna also develops this for Nico, at one point forcing herself not to tuck him and try to make him comfortable as he sleeps, because he is her comrade, not her little brother, and he wouldn't appreciate it. It goes out the window by the end of the book, where she goes out of her way to make sure he knows he has a home at both camps and promises herself that he will be recognized for his bravery.
In The Kane Chronicles, Carter and Sadie are this to each other. Sure, they bicker a lot, and their ideals and personality clash many times due to their estrangement, but beneath those are a strong familial bond of that of the Kanes.
In Coda, Anthem is very protective of his siblings, Alpha and Omega.
In The Windwater Pack Ashtree becomes Zephyr's "spirit sister", acting like his protective elder sister, and he returns the favor when she becomes Sín Nara, despite considering her his big sister.
This is a feature of the entire MudWing society in the Wings of Fire series. MudWings live in family clans composed of all the siblings from a particular nest. The first to hatch among them is the "bigwings" and official leader of the family, who lead and protect all of their siblings. Clay, the MudWing on the main characters' team, is The Heart for precisely this reason — he instinctively "bigwings" his teammates.
Darkstalker was also this for Whiteout, helping her calm her down when their parents start arguing and telling off some dragons when they insult her for interrupting their marble game.
In Daniel Woodrell's novel Winter's Bone,Big Sister Instinct is one of Ree's main concerns. She frequently worries about how to provide for her two younger brothers, who she is essentially raising.
Will says the reason Gabriel hates him so much is because he compromised the latter's sister's virtue. But then again, it's Will, so we don't know if that's true or not. As of The Clockwork Prince, it's apparently because Will rejected his little sister's advances (in his defense, she was a lovesick twelve-year-old) and broke Gabriel's arm in public.
Gideon Lightwood towards his younger brother, Gabriel.
In Angelfall, Penryn's whole motivation to infiltrate the aerie is to get her little sister back by any means necessary.
Jonas towards Gabriel in The Giver, despite his growing disillusionment with his Community, it's not until he learns that Gabe is going to be killed that he decides to leave.
In Dead Space: Catalyst, Jensi has spent most of his life taking care of his older brother Istvan who has an Ambigious Disorder that can sometimes turn violent. Unusually for this trope, Jensi's protectiveness of his ill brother is shown to be a bad thing. Jensi gets in trouble trying to protect Istvan, guilt over failing to protect Istvan from things Jensi could not control causes Kensi to live a depressed and meaningless life, and since this is Dead Space, Jensi extreme desire to keep his brother safe eventually lands him in the middle of space zombie outbreak.
In Vampire Academy, Adrian Ivashkov acts as a protective big brother to Jill Mastrano. She is fifteen-years-old, and kind of timid. He is an adult, several years older than her, and takes an interest in her well-being.
Journey to Chaos: Eric thinks of his teammates and Kasile as his younger siblings and is thusly protective of them. This is especially the case with Kasile because she is a queen and a target for Evil Princes. You don't want to get between an angry grendel and his siblings.
In the Relativity series, Michael takes it upon himself to protect his sister Sara from any potential boyfriends he doesn't approve of. Since she's an adult and can take care of herself, she's not happy about this.
Outlander and its spinoffs has Lord John Grey's older brother, Hal, who really cared about him and it's implied that he knew about his sexual orientation of being homosexual which, at that time, could get someone executed. John realizes this when he learned that Hal reassigned him to be the warden of the Adsmiur prison in Voyager just to protect from the scandal of being associated with a serial killer. In the eighth Outlander novel, he demands Claire Fraser regarding the whereabouts of his brother.
The Silver Codex: Dominique is very protective of her siblings to the point that scares even her.
Dreamblood Duology: Ehiru is supposed to take a role between big brother figure and mentor towards his apprentice Nijiri for the duration of Nijiri's apprenticeship, and act accordingly. However, the situation turns out complicated because while Ehiru does love Nijiri like a brother and has to constantly remind himself not to underestimate and coddle Nijiri, Nijiri's feelings and motives are more complicated. In the long run it's Nijiri who takes that role towards Ehiru, going to great lengths to keep trouble away from him and coddling him.
The Lord of the Rings has two prominent examples of this: Boromir, who actually looked after and protected his younger brother Faramir throughout their childhood, and Éomer, who was batshit protective of his younger sister Éowyn. The main reason Boromir volunteered for the trek to Rivendell boils down to this - he deemed he was better suited for it (which turned out to be wrong in the end). When Éomer found his sister deadly wounded at the Pelennor field, he and his entire army went into suicide death squad mode, sweeping all their enemies aside in the process. Clearly Éomer, in the moment of blind rage, found it hard to carry on knowing his sister probably had less than hours left.
In the Legendarium of J. R. R. Tolkien, aside from Lord of the Rings, we have Túrin, who was equally protective of his sister Urwen Lalaith, until she died in the plague only three years old. Word of God has stated that Túrin always had a weak spot for blondes after this, a Freudian Excuse for his protectiveness of Finduilas, and later for Nienor — who, as it turned out, was his sister. The main reason for his suicide, was the realization that he had ultimately failed her.
In Warrior Cats, Squirrelflight is very protective of her gentler sister Leafpool, will tell anyone off for verbally ganging up on her, and even adopts Leafpool's kits and raises them as her own (to hide the secret that they are a result of Leafpool's forbidden relationship.)
In The Amy Virus, Cyan's older sisters Annabeth and Tam treat her with much more respect and understanding than their parents do. For example, when Cyan's father tries to force clothes on her and hold her down so she can go to school, Tam instantly stands up to him and points out that Cyan actually threw up the previous night, so she was too sick to go anywhere.
Family Skeleton Mysteries: When Georgia was nearly molested as a child, this is what prompted Sid to come back from the dead and protect her - seeing her reminded him of his own little sister, who'd predeceased him.
In The Bridge Of Clay Matthew Dunbar, although barely 18 himself, has to look after his four younger brothers after their mother dies and their father leaves them. And he is very good at it.
Played for some tragedy in Broken Gate on Tora's end. In chapter 11, he remembers the time with his sister and he can feel Nezumi dying but can't do anything about it. Subverted, as would be expected on Ryuuji's end, as he did mistreat Nezumi, said mistreatment got him cursed.