Normally in works that center around Foolish Sibling, Responsible Sibling relationships, the older one is portrayed as the mature one who keeps their younger sibling in line, with the latter usually playing the Annoying Younger Sibling role to their Aloof Big Brother. But what happens if the roles are reversed?
In that case, you'll get the Mature Younger Sibling, whose main role is to act as the voice of reason to their older sibling, who in turn is either a Childish Older Sibling, Disappointing Older Sibling, a Big Brother Bully, or a combination of all three.
The younger sibling may be an Adorably Precocious Child or Wise Beyond Their Years, or maybe just an average kid themselves but has some sort of maturity advantage which causes them to claim seniority over their older sibling. The older sibling may be a Kiddie Kid who either acts in an infantile way due to a head injury as a baby, a mental disability, or perhaps they do know better but just decide to act immaturely towards their younger sibling For the Evulz. In extreme cases, the parents actually trust the younger sibling to look after the older one and babysit for them while they're gone.
To signify the ironic power dynamic, the younger one may be a Big Little Brother while the older one may be a Little Big Brother.
Contrast Childish Older Sibling, Disappointing Older Sibling, Annoying Younger Sibling, and Oblivious Younger Sibling. If the younger sibling has flaws of their own that cause them to be overpowered by an older person in authority, they're Only Sane by Comparison.
- Ui Hirasawa from K-On! is more mature, hardworking, and responsible compared to her older sister Yui. She acts as the parental figure while their parents are away, taking care of the household chores and Yui as well.
- From the Lucky Star spin-off Miyakawa-ke no Kuufuku, we have Hikage Miyakawa, a nine-year-old girl who is economically responsible and avoids wasting money in contrast to her older sister Hinata, an otaku girl who spends the majority of her paycheck on otaku merchandise and is the main reason why the duo is struggling in poverty.
- No Matter How I Look at It, It's You Guys' Fault I'm Not Popular!: Tomoki takes school seriously and does chores, unlike his older sister Tomoko, the socially dead otaku who would rather just play video games all day. He's even taller than her to symbolize that he's the more mature one.
- Pokémon: The Series: Unlike her air-headed Valley Girl older sisters (although Daisy goes through some Character Development), Misty actually takes battling seriously and often finds herself taking care of the Cerulean City gym.
- Downplayed with Rerun. His older brother Linus isn't immature per se (in fact, he's one of the most sensible characters), but Rerun doesn't suck his thumb, believe in the Great Pumpkin, or carry around a blanket. Rerun has admitted he doesn't idolize Linus in any way.
- Linus is a wise kid (albeit one with a slight immature streak) who is mostly even-tempered and in touch with moral principles, as opposed to his older sister Lucy, who will fly into a rage at the smallest provocations. He at least tries to steer her in the right direction, but she doesn't listen to him.
- While Miwa is the oldest of the Hamato siblings in Same Difference, it is stated multiple times that Leonardo is the most mature out of all of his siblings, often acting as the responsible older sibling towards all of them.
- In Onward, Ian is more level-headed, timid, and less enthusiastic about going into dangerous situations than his older brother Barley, who jumps right into trouble without thinking things through. This is actually the source of some conflict between them, as Ian considers Barley a Disappointing Older Sibling.
- Godzilla vs. Kong: Josh Valentine is apparently one. Although we never meet his Disappointing Older Brother, Madison's remarks about the guy indicate he's lazy and/or oblivious. Josh on the other hand is a sharp Cowardly Lion with moments of cunning, savviness and bravery when he's helping his best friend commit corporate espionage or trying to shut down an omnicidal Mechanical Abomination.
- Ascendance of a Bookworm: Ferdinand, one of the more straight-laced characters to start with, eventually turns out to have a Childish Older Sibling as an older half-brother. On top of this, it turns out that he's spoiling his older brother without realizing it and hiding some of the darker aspects of his past from him, which are situations that come across as more fit for an older sibling in regards to a younger one rather than the other way around.
- In Doctrine of Labyrinths, Mildmay is several years younger than Felix but spends the bulk of the entire series trying to mitigate the damage Felix's recklessness and mental health issues cause, to the point of even having to manage Felix's money for the pair.
- A Song of Ice and Fire: Amongst Cersei's three children, it's Myrcella Baratheon, the middle child, who's easily the most mature, especially for a child as young as herself. She's sharp and very strong-willed, in contrast to her cowardly, stupid and utterly vicious devil-spawn of an older brother, Joffrey.
- The title character of Blossom is the youngest of three, with older brothers Tony and Joey. However, as their father put it in one episode, "Tony has a head full of problems, and Joey... has a head full of hair." Joey is sweet but incredibly dumb, while Tony is more intelligent but also a recovering alcoholic. Blossom, with her interest in social causes and excellent academic record, is easily the most level-headed of the three.
- In Good Luck Charlie, Teddy Duncan is this to her older brother, P.J., being the wisest and most responsible of the Duncan siblings. While P.J. is much less of a troublemaker than their younger brother, Gabe is, P.J. is far more immature and airheaded than Teddy is, and far more careless than both Teddy and Gabe.
- iCarly: While Carly has a goofy sense of humor and often gets into scrapes with her friends, she's much more organized and mature than her older brother Spencer, who prefers to spend his time making abstract, borderline dangerous art pieces. She often comforts him when he's distraught. However, Spencer proves himself to be a good caretaker even with all his dumb ideas. In one Christmas episode, Carly lashes out at Spencer and wishes that he was normal, though she regrets this after an It's a Wonderful Plot where she sees how boring her life is with a mature older brother.
- Picky from EarthBound (1994) is immensely kinder and more mature than his older brother and Ness's childhood friend, Porky, is.
- Neptunia: Nepgear is this to Neptune, to the point where she's sometimes mistaken as being the older sibling of the two.
- Zigzagged in Rosebuds with Maricela. On one hand, she often needs to be looked after by her eldest sister, Rosa. On the other hand, she proves herself to be much more mature and responsible than her other big sister, Maria. She is also the smartest of the three sisters, which Rosa acknowledges and likes about her, but there are moments when Maricela may accidentally rub that fact in.
- Deconstructed in Mob Psycho 100. While Ritsu is good at assuming a Big Brother Mentor facade for Mob, having to carefully manage his older brother's fragile, literally explosive emotional state for so long wears him down and leads him to silently fear and resent his brother for robbing him of a healthy childhood and sibling relationship. So when he finally manifests his Puberty Superpower, he quickly develops a toxic strain of Chuunibyou and starts lashing out at nearly everyone he encounters, culminating in an angry rant at Mob and a challenge to a psychic duel.
- The Amazing World of Gumball Anais Watterson is a Child Prodigy, (more often than not) the Only Sane Man, and baby sister to her trouble prone Idiot Hero brothers, Darwin and Gumball. An early episode where their mother, against her better judgment, leaves Gumball in charge of the house while the kids are alone, has Anais point out (and her brothers don't deny) that she basically helps raise them. Though it's somewhat balanced out by her having less social skills. Darwin can be considered a downplayed version of this to Gumball too, as he's technically a few years younger and he often acts as the more sensible of the duo. Though intelligence wise, they flip flop on who's smarter if either and Darwin's functionally treated as the same age as Gumball.
- Avatar: The Last Airbender: Katara is generally a lot more serious, motherly and authoritative compared to her older brother Sokka, who starts off the series as a childish goof. Sokka even comments that her Team Mom role in his life since their mother died has gotten to such a point that he can only recall his little sister's face in place of his mother's.
- Bob's Burgers: Louise Belcher views herself as this to her brother Gene, who she terms her "larger brother." Keep in mind this is more in terms of relativity as Louise herself can be fairly immature.
- In Dexter's Laboratory, eight-year-old Child Prodigy Dexter plays the Only Sane Man role to his ditzy older sister Dee Dee, who is stated to be in the sixth grade, putting her age somewhere between ten and twelve. Despite this, she constantly invades Dexter's secret lab after several warnings from him to stay out, messes with his inventions without knowing how dangerous they are to the inexperienced, cries over being snapped at, and makes sarcastic remarks to Dexter whenever she's not being stupid.
- On Family Guy, we have Chris and Meg, two teenagers, the former of whom is an immature Dumbass Teenage Son and the latter who is a Bratty Teenage Daughter with serious anger issues. Of all three Griffin children, Stewie, who's just a one-year-old baby, is the closest to mature, usually bringing up valid points whenever his older siblings are acting up, which they usually ignore since the family normally can't hear him. However, considering that Stewie's a matricidal Enfant Terrible who's committed many heinous acts, insults his family, and occasionally uses Brian for personal gain, he's Only Sane by Comparison.
- In Foster's Home for Imaginary Friends, we have eight-year-old Mac and thirteen-year-old Terrence. The latter would be expected to be the mature one, but it's quite the contrary actually: Terrence only exists to ruthlessly torment Mac and his Imaginary Friend Bloo physically For the Evulz, being quite the Big Brother Bully and sociopath. He's also extremely stupid, not being able to tell apart red from green, thinks Singapore, Malaysia, and Wisconsin are all the same country, and he falls victim to slapstick on occasion as a result of his clumsiness. And to top it all off, their mom never believes him whenever he makes up stories playing the role as the victim. That said, Mac is trusted by their mom more than Terrence; he very easily outsmarts Terrence in his arguments and usually avoids the fall for his actions, with Terrence usually facing karma in the form of slapstick.
- The title character in Kick Buttowski: Suburban Daredevil, while not exactly academically gifted and prone to screwing himself during his stunts at times, is still much more responsible and level-headed than his boneheaded, disgusting older brother Brad.
- The Loud House:
- 11-year-old Lincoln, the only boy in the family, usually serves as the Only Sane Man among his circle of ten sisters, who, both the younger and older ones, usually start fights with him, blaming him for their own faults, and rub their personality quirks in his face while he's trying to go about his day. Sometimes he'll be the one to cheer up one of his older sisters whenever they face a problem.
- Lisa is the most intelligent of the Loud siblings despite being the second-youngest and frequently uses Sesquipedalian Loquaciousness, as opposed to her other siblings' more casual diction. She's also very calm, which makes her older siblings seem more immature, as they're all rather emotional (besides the similarly stoic Lucy). Played with to an extent, though, as there are several things she doesn't know how to do without help from an adult, given her young age.
- In The Owl House, twins Edric and Emira Blight are both unapologetically irresponsible troublemakers (though Emira generally has more common sense than her brother), while their younger sister Amity is serious, well-behaved, and constantly trying to rein them in.
- Regular Show: Don is a chill guy who does well at his job as an accountant, as opposed to Rigby, the goofy Manchild who would rather mess around than work. This, in addition to their height differences, was what got people to mistake Don for the older brother (much to Rigby's frustration).
- On The Simpsons, eight-year-old Lisa is much smarter and wiser than her ten-year-old brother Bart, often warning him against bad ideas and such forth. In fact, in "My Sister, My Sitter", when the parents are out, she is said to be babysitting him rather than the other way around.
- Steven Universe: Peedee is a Workaholic who has a rather pessimistic outlook on life, as opposed to his older brother Ronaldo, who has a lot of energy to spare and doesn't really care about working.