In Alvin and the Chipmunks: The Squeakquel after Simon nearly drowns in a toilet after some bullies gave him a swirly, Alvin is ready to go get revenge, but Simon tells him that violence will not solve anything. It isn't until they see the same bullies picking on Theodore that they both attack the bullies like rabid animals.
Aussie horror flick Black Water is actually rather sweet in spots due to it's use of this trope. Initially, we have both Grace and her boyfriend Adam towards Grace's shyer, more panicked sister Lee. However, Lee turns out to have a Little Sister Instinct herself. You see it emerging slowly throughout the film, but once Grace is injured, Lee is willing to face down crocodiles if it means she can get her sister out of there and to a hospital. Turns out she's just much braver for her sister's sake than she is for her own.
The Blind Side: With only a split second to act, Michael instinctively saved adoptive little brother SJ from being seriously injured or killed during their car accident.
A very dark version in Cafe Setareh: Khosro hates his sister's abusive husband so much that he picks a fight with him and unintentionally kills him, which directly and indirectly leads to all of the characters' lives falling apart.
Becca from The Descent is very caring and protective towards her sweet little sister Sam, and to a lesser extent, the rest of the less-experienced caving group. Also, it's fairly subtle, but Beth displays Big Sister Instincts towards anyone in the group who needs it — she's constantly looking after the emotionally fragile (due to the death of her husband and child) Sarah, she gives her jacket to Holly after she breaks her leg and helps her stand and walk (along with Sam, The Medic) and she goes back to help Juno in the crawler fight, even though she doesn't like Juno much.
In Doom, the Staff Sergeant John "Reaper" Grimm fiercely protects his sister Samantha from a group of mutant monsters.
The titular character of Ginger Snaps is an interesting example — her protectiveness towards her younger sister is essentially, a snarky sense of humour aside, her only redeeming quality, and that's pretty messed up. They have a suicide pact ("Out by sixteen or dead on the scene,") and her gradual case of lycanthropy worsens, she starts killing people if she thinks they even looked at Brigitte the wrong way. She's enjoying killing more and more, so this trope becomes more of an excuse in the end. However, Sam could be seen to have a genuinely altruistic Big Brother Instinct towards Brigitte, as he gives her an awful lot of help, carrying a fair amount of risk, with no benefit whatsoever to himself. Although some people interpret it as him having a romantic interest in her.
Sonny Corleone launches into one of the most epic No Holds Barred Beatdowns in film history against his brother-in-law after he finds his sister with a black eye. The enemy family Barzini later uses this instinct against Sonny by paying Carlo to deliver a savage beating to his wife in order to easily set up an ambush for Sonny.
Michael gets his turn when he has Carlo whacked in the finale, avenging his older brother and ending his younger sister's torment. Subverted when Michael has Fredo assassinated for working against family interests.
A somewhat disturbing and sweet take on Michael and Laurie in Halloween (2007). It's shown very clearly in the beginning that Michael loved his baby sister. Even though he murdered just about everyone in the family, he left her alive. This also applies after he breaks out of the mental institute and seeks her out. Unfortunately, when he does meet up with her, he's practically a mute and he's killed all of her friends.
The Hunger Games: Katniss Everdeen is deeply devoted to her beloved sister Prim, and she volunteers to take her place in the Hunger Games after Prim's name is called. This is essentially a death sentence and she's fully aware of it. When she's in the arena, she forms an attachment to Rue and becomes her steadfast ally. She's entirely heartbroken when Rue is killed, singing to her as she dies and burying her in flowers, which in Panem is akin to flipping the bird against the Capitol, who only see the tributes as mere entertainment props.
And that's without mentioning what she do when her own sister, Prim, is killed in the final book. Despite not trusting President Snow at all (he DID razed her entire district to the ground), she makes damn sure that whoever ordered the bomb that killed Prim would pay the price. Bye, President Coin.
A consistent theme in Jeepers Creepers. In the first film, Trish tries her damnedest to convince the Creeper to take her instead of Darry. It doesn't work. And in the second film, Jack Jr. tries to chase down the Creeper when it takes his little brother and then joins their father in a Roaring Rampage of Revenge after Billy's death.
Labyrinth promotes big sister instinct. The entire movie revolves around Sarah trying to rescue her baby half-brother from the Jareth. The manga sequel has more of this than the film, though.
In the extended edition, there's also a scene where Éomer finds an unconscious Éowyn after the Battle of Pelennor Fields. He holds her and cries, before getting her medical attention.
Boromir to Faramir as well, in a more uncommon male-male example. Boromir is quick to defend and praise Faramir in the face of their father's scorn and dismissal of his younger son.
Ouija: Laine to her rebellious younger sister, Sarah, who is a bit annoying. Since their mother's death, she has practically taken the role of mother, be it forbidding Sarah to come out late at night, or stopping an unknown man from taking her to a date. Then the two become the only survivors of the curse and Laine's Big Sister Instinct comes to its fullest when she prevents the Eldritch Abomination from strangling her sister by inviting her for a game of Ouija.
A fairly central motif throughout The Proposition. Charlie's motivation throughout the entire film is to protect his younger brother, Mikey. To do this, he may need to kill his older brother. The oldest brother, Arthur, wants nothing more than to protect both of his younger brothers.
Averted, though, in the Spiritual Sequel, Lawless. After Jack gets brutally beaten by a Corrupt Cop, his older brothers basically tell him that he has to learn to fight his own battles.
Star Wars: He's only the big brother by about six or seven minutes, tops, but Luke Skywalker definitely qualifies. He's all redeeming and peaceful and unflappable in the face of evil despite a million reasons not to be, but all bets are off when the Big Bad makes an impotent, hinted threat towards his sister. Suffice to say, he doesn't react well.
Han Solo to Luke Skywalker. Despite always calling him "kid" and having doubts about Luke's plans, Han becomes fond of Luke early on and often goes out of his way to protect or rescue him. In "Empire Strikes Back", he risks his own life by going out into dangerously low temperatures to find Luke after he doesn't return to the rebel base.
Peter Garret in Vertical Limit climbs up K2 to rescue his sister, Annie.
A variant in Warriors of Virtue with Elysia for Ryan. She's initially seen on the good guys' side for a third of movie, but then it's revealed that she's a mole for Komodo. When Ryan is captured, she tries to make him understand her and dies because she prevented Komodo from killing him.
Seen in West Side Story when Bernardo interferes in his sister Maria's budding romance with Tony. Double Subverted in that Tony is the only decent member of the Jets, genuinely in love with Maria and with good intentions... but then he ends up killing Bernardo in a fit of rage over the death of his friend.
Big Sister Instinct is one of Ree's main motivations in Winter's Bone. She is shown to be constantly worrying about how she'll provide for her younger brother and sister if they lose their house.
Trampled on in Wishmaster. As soon as the Djinn threatens Alexandra's sister, she states point blank that she'll kill him if he harms her. Amused, he shows her his true face to show how little threat she poses to him, and she can only respond with a terrified "oh my god".
For all his (many) flaws, Victor Creed really loves his little brother Jimmy. It's best illustrated in the Civil War part of the opening montage, when Logan is shot and they are (at the time) unaware of their healing factors, Victor's expression says it all.
Kayla appears to be willing to do anything to keep her sister safe.
Charles had this for Raven, being very concerned for hiding her mutant form in public and telling her directly that he didn't want anything to happen to her. However, his relationship with his foster sister deconstructs this phenomenon because it caused him to become overprotective to the point where he denied her the chance to embrace her true appearance and be proud of what she was, as well as caused him to ignore her very obvious crush on him. This in turn made Raven turn to Erik as a mentor/love interest and eventually join him in his cause against humanity.
Darwin develops this for the younger mutant recruits, especially when Shaw attacked the CIA base. Unfortunately, this only gets him killed by Shaw when he tries to protect Angel from him.
By the time of the Cuban Missile Crisis, Erik had already begun to view Charles as a brother figure. When the Blackbird spiraled out of control, Magneto used his body to shield Xavier from injury, and he immediately halted his attack on the American and Soviet naval forces when Charles was shot.
When the younger Charles sees Mystique again for the first time in 11 years, he caresses her hair and shoulder as a gesture of comfort while telling her that he will keep her safe among other soothing words. When Magneto then points a gun in her direction, Xavier stands in between the barrel and his foster sister, and Erik later has to use his power to change the bullet's trajectory so that it won't hit Charles in the head. Despite their estrangement, Xavier is willing to die for Raven to save her life.
Peter Maximoff is seen playing with his little sister. He's even willing to sit down and watch television with her, which is heartwarming considering how hyperactive the guy is.
Alex Summers brings Scott to Xavier's school in the hope that his former mentor can help his younger brother cope with his potentially dangerous mutant power.
When Apocalypse is asphyxiating Mystique, Charles doesn't even think twice before he expresses his willingness to surrender himself, but then Moira reminds him that he can't do this because the entire planet would then fall under Apocalypse's control. Xavier collapses in tears when he realizes that he can't exchange his life for his foster sister's.
Beast behaves this way towards Cyclops. He designs a means to help the young man see without vaporizing everything. During the final battle, the two are constantly by each other's side, which is especially poignant considering Scott's actual older brother and Hank's former teammate Alex had died earlier.
Once Magneto pulls a Heel–Face Turn, he stretches his powers to their limits, doing everything he can in order to protect Charles, whom he loves like a brother, from Apocalypse.
Zathura: Walter originally acts aloof and is somewhat a bully towards his younger brother, Danny, such that the astronaut whom they meet call him out on this. Then it's revealed that the astronaut is in fact Walter all grown up who used his wishing star to wish Danny disappear in a fit of rage, which he has thoroughly regretted because that means he is stuck in the space forever. He then tries to atone for this by going to the past and influencing Walter to start treating Danny as more than a playtoy. Once he learns about this, Walter softens and becomes genuinely caring to Danny.
Their older sister, Lisa, is this too, to a lesser extent. Contrasting with her aloofness and obliviousness to the situation, the bit where she's pulling Walter away to safety from the titular Zathura in exchange for getting sucked herself near the end of the film is kind of sweet.