Okay jc is the baby of the family, but not really. Nhs and lwj have really spoiled baby brothers attitude, but jc has an elder brother complex in my opinion and this makes me insane. I don't know if it's because he is the sect heir or because of his family dynamics. But these are my thoughts, yours?

With the understanding that I can only speak to CQL (the novel wasn't to my taste, personally, so I didn't get far in it) I am FASCINATED by your idea because I have a very different interpretation of my own!

For context, I lack the relevant cultural elements, so I don't think I can comment knowledgeably on some things? But I, who has some very similar personality traits to JC, am the youngest of 4 in a family dynamic that's similar enough to the Jiangs that, more than once, I had to pause CQL and touch some grass outside. 😅

Personally, I read JC as the Quintessential Youngest Child (TM) in the sense that, like:

He kinda takes his older siblings' support for granted, which is a) common for younger siblings, who have never known life without siblings, and b) JC's strongest and most immediate form of emotional support (since he probably can't/won't connect much with the other disciples and his relationships with his parents are rather fraught). It also means that where he might be more suspicious of other people, he often doesn't even question his own siblings (read: WWX) even when faced with evidence that maybe he should. It's a specific kind of naivete that I expect to see more often from younger siblings who have internalized an "us vs. the world" kind of dynamic.

Which also means that we youngest sibs can be ABSOLUTE DISASTERS when we lose that support, or think we've lost it. (JC being a little bitch during Lan indoctrination was a mirror of me when one of my older sisters started dating boys, how dare she have a life of her own that didn't involve me in every aspect!!! Clearly this means I have to be a passive-aggressive bitch about it!!!!!)

The jealousy, born from insecurity, of seeing older siblings be more liked/talented/etc than you, which you're too young to understand is the result of differences in age and maturity with the accompanying personal growth. No, it's clearly because You Suck.

....and then compound that inferiority complex with the pressure of being The Heir. In my family, I was the child of both parents while my siblings are half-sibs via my father; my mother was not unlike Yu Ziyuan in terms of her Expectations For Me, but my Jiang Fengmian-esque father favored his eldest son. So you push yourself to overachievement and hold yourself to an impossible standard to Prove You Matter As A Person. But JC, who is an accomplished cultivator in his own right (e.g. he developed his golden core a year early!), is in the shadow of a brother who's a legit genius, so what efforts might have 'worked' in other families won't work here. ('Worked' is in scare quotes because this isn't a healthy perspective to have, obvs.)

And when you're the baby sib, you're never not the baby sib, and damn but this can give you such a complex lmao. I don't know how many times I've had to remind my siblings and surviving parent that I'm an adult, I've been an adult for a long time actually, I'm allowed to make decisions about my own life, and I am in fact capable of saying Grown Up Things. It's infuriating! But when you get angry about it, it's just a sign of your immaturity! So then you're labeled as too sensitive, too reactionary, can't you just calm down... Look, at the risk of self-projecting too much, I'm just gonna say that I don't always like JC or agree with his decisions, but holy shit does his character land on some real personal places for me lol. ANYWAY, WWX's refusal to tell JC about the golden core exchange sounds to me, as a baby sib who clearly overidentifies with JC, like, "I don't trust you to make the right decision for yourself, so I have to do it for you, even though we've survived enough things by now for you to have proved yourself an adult capable of making your own choices." But the golden core thing is enough of A Thing to unpack that I'm just gonna leave it there with the clarification that this isn't actually meant to be a value judgment on WWX's action either way.

Also, 'mixed households' in regards to children who are adopted vs. blood-related can have some super complicated emotional dynamics, but this point starts getting into some cultural stuff that I don't feel I understand well enough to try digging into, especially with how horribly messy Anglophone fandom has been regarding the nature of WWX's place within the Jiang family.

SO YEAH ANON that's kinda the main points of my take on why JC Is Family Baby Whether He Likes It Or Not and some of why WWX Has Permanent Big Brother Disease lmao. I try to stay grounded in what I think I see actually on the screen and how it impacts the characters, but of course it's inevitably informed by my own experiences, for better or worse.

But that doesn't mean your take isn't just as valid or maybe is even based on the same points, I could totally see that happening depending on what you mean by 'elder brother complex.' ヽ(✿ﾟ▽ﾟ)ノ