The Incredible Hulk: It’s a usually Downplayed Trope but Bruce Banner aka the Hulk still cares for his cousin Jennifer Walters especially in his Smart Hulk transformation in the John Bryne era. Hell Bruce saved Jennifer’s life with a blood donation when she got shot and unintentionally made her incredibly badass. Case in point, Banner Hulk was getting his ass kicked by the Killer Robot Arsenal who had previously required an entire team of Avengers to subdue. Jennnifer rushes in to help her cousin only get swiftly knocked out. Cue the Hulk's signature Unstoppable Rage-fueled No-Holds-Barred Beatdown.
There’s also the inverse: Shulkie tends to believe she’s the only one who can reach her cousin when he’s on a Unstoppable Rage. The quickest way to piss She-Hulk off is to hurt her cousin especially when he’s in his weaker Bruce Banner form.
Besides Jen, Hulk does share the same sentiment with his sidekick Rick Jones and surprisingly Spider-Man, who he even lets take a ride on his shoulders and will get very, very mad if “Bug Man” is hurt around him.
Black Panther: T'Challa does not take violence or insult towards his younger sister Shuri well at all though she can usually look after herself well enough. In more recent comics, he’s gone above and beyond to bring her back from the spiritual afterlife.
Captain Britain: Brian Braddock is protective of his twin sister, Betsy, even though these days, she's plenty capable of taking care of herself. He's also shown as being particularly protective of Rachel Summers and Kitty Pryde during their time on Excalibur, when both were teenagers. In Rachel's case, this involves dealing with/trying to deal with creeps on at least two occasions — ironically, both were technically same guy (or rather, alternate versions of the same guy, Nigel Frobisher, Merchant Banking). The first time, he pulls a "your lips are saying that, darling, but -" and Brian promptly looms up behind him and says, "So do mine. Go away." The second time, he's admonishing her for literally tarring and feathering the local version of Nigel Frobisher when they're in another universe. Then she shows him what alt!Frobisher was thinking. It isn't revealed what that was, but it was bad enough that it makes Brian go from issuing a stern rebuke of Rachel in one panel to forcing his entire team to hold him back from beating alt!Frobisher into paste in the next.
Fantastic Four: Susan Storm is a very compassionate woman, but god forbid you hurt her baby brother Johnny because those force fields have beaten Marvel’s strongest heroes and villains. A great example of this was when the Grandmaster forced Sue and Johnny to fight to the death, Sue, through sheer force of will, defied Grandmaster, refusing to hurt her brother no matter what. When Johnny died (he got better) in a Heroic Sacrifice against Annihilus‘s army, Sue was utterly devastated and put a force field around herself, refusing to let anyone including Reed and her children near her.
Reed and Ben also don’t take harm towards Johnny lying down and have got particularly violent when it comes to his safety. Reed, a generally peaceful man, even considered using the Ultimate Nullifier on Annihilus in revenge.
Johnny himself has this towards his nephew and niece, Franklin and Valeria, though it overlaps with Papa Wolf more often than not.
Ghost Rider: The third Ghost Rider, Robbie Reyes, is the legal guardian of his little brother Gabe, and will gladly and quite literally unleash Hell if it keeps him safe.
The Mighty Thor: Thor treated Loki this way in early flashbacks to their childhood; however, it only aggravated Loki's already raging inferiority complex, putting him on the path to villainy.
However, after Loki accidentally brought about the near-total destruction of Asgard, was killed in a last-ditch heroic sacrifice to save what was left (that may have been part of The Plan) and reincarnated as a kid with only his childhood memories, he appreciates this protection a lot, since the only reason most people in Asgard haven't killed him yet is because they know Thor would go on a Roaring Rampage of Revenge on them and the city if Loki is harmed.
Kid Loki seems to be similarly protective of Thor. During the Fear Itself event, he engages in dark dealings and agrees to restore the Ragnarok Cycle all to save Thor from being killed by The Serpent.
Thor has always been like this towards Loki even when the latter was a villain. For better or for worse, Thor sees Loki as his responsibility. Thor will always foil Loki's evil plots and step in to save him from worse threats.
The Punisher: In The Punisher MAX story "The Tyger" which details Frank's childhood years, one of Frank's best friends is raped by the son of local mob boss, and she killed herself shortly thereafter. Frank (who is not yet even a teenager) steals his dad's gun and sets out to kill the rapist. He's beaten to it by her big brother, a Marine, who lures the man away from his bodyguards, beats him half to death, then tosses him in an open grave, pours a can of gasoline all over him, and sets him on fire.
Spider-Girl: May Parker adores her literal baby brother Benjamin aka “Benjy” and will do anything to protect him, including travelling across multiple universes to save him. One time when the Carnage Symbiote latched itself onto Benjy, May was forced to use Ultrasonics on him which damaged his ears, at which May was stricken with guilt and blamed herself.
From the original X-Men Hank McCoy aka Beast cares deeply for his fellow less strong teammates, he even gave Iceman a spine cracking hug upon discovering he was alive. Hank is also protective of Jean though his feelings weren’t always brotherly. Later in the 90s Beast insisted on joining Gambit and Rogue’s date alongside Wolverine and Jubilee making sure Gambit “treats her right”.
Colossus, Gentle Giant he may be, anyone foolish enough to hurt his little sister Magik is in for a world of organic steel pain.
Both Colossus and Nightcrawler invoke this towards Storm, they even saved her from Dracula once. Nightcrawler even made Storm’s room more African so she’d be more comfortable.
Between Wolverine's kids, Laura is one of the few people Daken openly cares for, and she is likewise protective of him.
Still gender-inverted, but otherwise played straight between Laura and the Sisters — four clones of her created by Alchemax Genetics — in All-New Wolverine. She goes through great lengths to protect and help them, nearly loses her cool when she thinks Taskmaster killed them, and is visibly upset when Doctor Strange admits he can't help her cure the girls despite her assurances she would get them through it.
Honey Badger/Scout is very protective of her big sister, X-23. If you so much as threaten to give Laura a bad day, Gabby will come for you.
Anyone who messes with Nightcrawler’s sister Rogue is in for a Teleport Spam. On the flip-side Rogue cares deeply for her adoptive blue brother and once carried him in her arms to save him falling off a waterfall.
Gambit’s first canon appearance was looking after Kid Storm and same as Wolverine he explicitly cares for Jubilee and X-23 greatly.
Due to being older than them Cable is quite protective of his alternative universe counterpart siblings Rachel Summers and Nate Grey, although his interactions with Nate were initially pretty violent.
Separately, Rachel was very protective of Cable when he was growing up, forming a psychic bond with him as a baby that led to her going full Phoenix during Inferno and hurtling over to New York to try and protect him. She also, in the Mother Askani timeline, arranged for Jean and Scott to come to the future to raise him.
Nate was also protective of Cable at times, with the flashbacks in the 'Metus' arc of Cable's 2017 series showing Nate nearly getting himself killed to warn his older brother that Metus was coming for him. More generally, despite his often gruff demeanour and occasional Well-Intentioned Extremist/Anti-Villain moments, he's always a Friend to All Children and acted very brotherly towards his first cousins once removed, Gailyn and Joey Grey-Bailey, protecting them from the Prime Sentinels during Operation Zero Tolerance, including using the last of his powers to create a construct of Joey's bear.
Though they can find it hard to get along, Cyclops still tends to treat Havok this way, insisting on carrying his coma-ridden brother into the X-Mansion despite the presence of a wheelchair and being so protective and interfering that they're mistaken for a couple in an issue of X-Factor.
X-Men also has a lot of sister versions of this: Jean Grey to Storm, Storm and Jean to Kitty Pryde, Storm to Rogue, Rogue and Jean to Jubilee and Jubilee herself to X-23. Kitty Pryde ironically acts like this to the time traveling younger original five X-Men (specifically Jean, which is quite the role reversal compared to the 80s).
Downplayed but once in a blue moon Professor X and his stepbrother Cain Marko aka The Juggernaut have shades of this of this. Although they’re fierce enemies, deep down they still care for each other quite bit, due to growing up and suffering abuse together. Notably Charles tried to stop Marko from touching the Gem of Cyttorak and even when attacked by him Charles never wishes harm upon his brutish stepbrother despite being given many reasons to do so. Jugs himself has protected Charles and his school from World War Hulk, though he would sooner die than admit he actually cares little for his stepbrother and threatened those who dared suggest it.
Zig-zagged in The Avengers, when Thor begins to defend Loki but thinks twice after hearing how many people Loki has killed:
Bruce: I don’t think we should be focusing on Loki. That guy’s brain is a bag full of cats. You can smell crazy on him. Thor: Have care how you speak! Loki is beyond reason, but he is of Asgard and he's my brother. Natasha: He killed eighty people in two days. Thor: He's adopted.
Like in the comics, Pietro Maximoff acts as this for his twin sister Wanda in Avengers: Age of Ultron. The minute she is taken out by Hawkeye, he carries her off to safety and furiously promises to murder him for her sake.
Black Panther — insult or threaten T'Challa's sister Shuri, and you are guaranteed to get your ass kicked... as Killmonger discovered the hard way.
In Infinity War, Gamora reveals the Soul Stone's location to Thanos to stop him from torturing her sister Nebula.
Nebula returns the favour in Avengers: Endgame as when her evil past self aims her gun at Gamora, Nebula shoots herself down without hesitation.
The opening flashback of Black Widow (2021) shows that Natasha Romanoff was extremely protective of her "younger sister" Yelena Belova. When their mission to pose as an American family ended, Natasha, who was eleven years old, grabbed a gun off one of the soldiers that came to separate them and threatened to shoot if they tried to put Yelena in the Red Room again. When they reunite as adults, Natasha's protective instincts kick in again and she's seen telling Yelena to put a seatbelt on during a car chase.
In Spider-Man: No Way Home the Raimi and Webbverse Spider-Men display this to their younger Alternative Self from the MCU as well as Ned and MJ (the latter of whom Webbverse Peter saves from falling). The older Peters do their best to impart what they’ve learned to MCU Peter and look out for him, with the Raimi Spidey personally stepping in to prevent his young counterpart from killing the Green Goblin in revenge for Aunt May, Webbverse Spidey even admits touchingly that he always wanted brothers as a only child.
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.
Wolverine often acts as a mentor to one of the younger heroines on the team: Kitty Pryde (comics), Jubilee (comics/animated series), or Rogue (film/second animated series).
as a telepathic projection. Charles even apologises to Cain for being barred from joining the X-Men in Kraoka but also tells his stepbrother that he believes in him and knows he can turn over a new leaf, which Cain does.
