My main characters Bobbie and David are brothers. I try to write them as realistically as possible, considering I have 2 siblings myself 
David played a huge role in raising Bobbie though, so their dynamic is a mix of siblings and parental. It's interesting to write for sure.
For anyone who wants to check out my comic about these brothers 
I grew up with a lot of siblings. I think that is why a lot of my characters have at least one sibling. As well as dynamics like, sister-in-law and adopted sibling. Even in the case with Ken, an only child, there is sort of a brother dynamic with his foster cousin.
I guess the weird thing is that I have little desire to make a character with a lot of siblings, similar to my own family. Tho in an earlier version of Frostpeak West, Ace had like 4 or 5 half-brothers but I ended up changing his backstory. I guess it's easier to keep track of 1 sister. In earlier concepts, Ken was Ace's younger brother.
I honestly have very few stories where there isn't at least one relevant character with a sibling that has at least a little bit of bearing on the plot. I guess it's just a difference between writers, as everyone's experiences affect how they write and imagine worlds. My siblings are my best friends, so I guess it comes through with how I write family dynamics. Even if parents are conveniently absent for the sake of the plot, there is usually at least one sibling. My record is currently six siblings (five brothers and a sister)
Siblings can have unique back stories to each other though. Even when they are raised together, they'll have different experiences in school, struggles, xyz.
I realize that; that's why I put "unique" in quotations. ^^;
The difference is that, 9 times out of 10, a sibling will have some awareness of what's going on in their fellow siblings' lives. They may not know that they were suicidal in high school, for instance, but they'll be able to see their lonely daily activities and their negative attitude while at home.
If anything, if they DON'T notice, it would likely become a point of contention in the story, like a "you call yourself my brother, but you were never there when I needed you; you didn't even care!" sort of thing.
The point is, besides not getting to do things like inventing a brand-new country or family history or fantasy race or social dynamic along with the introduction of such a character, there won't be as many "new" revelations involved in divulging their backstory: there's always going to be that large probability that their sibling is already familiar with what you're about to reveal, and a necessity to write with that familiarity in mind.
Ahh, sorry, guess I just misunderstood the quotations. I would agree there's a good chance of siblings having some inclination what you're going through in a lot of instances, though that likelihood decreases the older the siblings are. (As in my story, "Damsel in the Red Dress" where they are both adults, and her little brother, though they are close emotionally, lives in Mexico, so all sorts of things go down in her neck of the woods in the first book without him even knowing.)
I personally feel like siblings is a dynamic so specific that no other person could possibly fill it (unless they were raised with you, making them your sibling imao):
"There’s nothing there but a Christmas card from Andrew and Jinho, ostentatiously addressed to “Her Majesty, The Queen of Baltimore.”"
*
“Jinho told me to say hello to the Queen for him. Gosh! It’ll be so awesome to see you again. You have no idea how much I’ve missed you since the move. He always says I talk about you like you’re some long-deceased relative and not just a phone call away. But calls are just different. We’ll probably be able to get lunch somewhere, the four of us, at least once before we leave. You, me, Jinho, and Kat. I just realized we haven't even introduced them, yet -"
*
“I’ll text you when we’re back in the country so we can arrange to get lunch - but hopefully sooner if we can. Our schedule has been hectic. P.S. I saw the news report for your exhibit online. I’m totally telling everyone that my older sister is a celebrity! (Jinho says we have to fix your house so we can start name-dropping XD.)”
*
It's just different fam
My story has two important characters who are siblings and their dynamic is an important part of the story.
My story has a protagonist, who saves his sibling from fae hands They are both positive characters (even if overall the novel has a very dark mood).
As an older sibling I feel like light duties are common (from 13 I was "in charge" of my younger brother while my parents were out, which usually meant "don't fight with him and make sure he doesn't leave on his own") but proper psychological "parentification" is a lot rarer as it's a form of abuse to force an older sibling to take on the same duties as a co-parent from a young age. I'm talking being responsible for a much younger siblings food, health, cleanliness, etc regularly to the point the sibling is dependent on you rather than the actual parents.
Like I'm an older sibling but I still had a childhood and space to explore who I am as an independent being from my parents and my brother rather than needing to put my brother first because I'm his carer, and I'm not particularly more responsable than him at heart (I'm an anxious bean who's a workaholic though so sometimes it looks like it). He also often comes off as the more responsable one because he presents as this very serious socially awkward tech dude meanwhile I'm the coloured-hair, delirious from overwork artsy one. More than once at gatherings with extended family people have assumed he's older than me.
Yes, light duties are entirely normal and common. But that wasn't really the case in my childhood, as when my mother was deported, I had to care for my younger sister (specifically haircare) because she was too small to do so herself, and my father has a hand injury that makes it tend to lock up. And before that my mother and I had moved without my father and older brother so I spent a very large amount of time caring for the household mostly alone while my mother went out to deal with other work.
I'm so sorry you went through that 🫂 deportation as a whole is a cruel industry, I hope you've been able to reunite and are safe.
I can totally see why in cases like that the older sibling steps up into a parent role despite it not being fair on them, and the parents letting it happen despite probably not wanting to have to put the kids through that.
With my main siblings in "Damsel in the Red Dress" I really wanted to express the struggle of an older sibling in really trying life situations trying to protect their younger siblings from the reality of their lives. My siblings and I have gone through pretty trying circumstances, on memorable night being sleeping in an abandoned building. These emotions of terror, and sadness while trying your best to comfort your younger siblings are the sort of feelings I imagine Alicia having for Andrew as she tried to shelter him from the reality of their parent's divorce as much as she could
Spot-light on siblings from large families:
Shannon Carmichael from Damsel in the Red Dress has several siblings, coming from a very large American-Islamic family. Though they're only mentioned in passing, he's the only boy and the only one of his siblings to have gotten a degree
My comic has sibling pairs, with one of them being a pretty big spoiler, and the other being Hugo and his sister. The spoiler siblings are pretty dysfunctional, but Hugo and his sister are pretty fine with eachother even if they’re not exactly best friends.
My siblings in both "Hushabye Prince" and "Damsel in the Red Dress" are pretty close, but things are very different for both sets. Both Alicia x Andrews and Jinwoo x Jimin love each other to death, but one set is complicated by a divorce that meant growing up separately and the other by...spoilers lol
