1. How parents treat/behave with kids, among themselves and others are observed and adopted or disliked by kids.
2. Bullying in schools are another one.
3. How elder kids treat the younger one.
4. Beyond that, every kid has his/her own nature and character by birth. All Siblings are different despite having same parents, same upbringing, same environment at home.
But we don't all have the same home environment. There are generally some years between births, and things happen in those years. Good things, bad things, and things. It'll be more pronounced if there is more time between children.
I mean, just myself: I have two siblings. One is 6 years younger, the other 11 years younger. Between me and the older of the two: My dad graduated college and got a decently stable job. My parents moved away from my mom's family, who helped take care of me while my mother worked, and moved to a small town. My mother started staying at home since the finances allowed it.
We moved yet again before my younger sibling was born: My father switched careers and made more money than before (which was rich compared to their finances while I was young and my father was in college).
All this meant that we got attention from different sorts of folks while young. Our parents had different finances: While I had poverty while young, my youngest sibling had nothing of the sort. My parents were more experienced by the time my brother was born, too. We all had different sorts of challenges which definitely affected the ways my parents communicated with us - their continuing maturity did this as well: I was born before either were 25, but my siblings after.
But what OP refers to is that kids are born with their own personality which already emerges from a very young age in the first year.
I've seen it with my kids, and also other young kids in my close environment. For instance, some are just naturally extraverted, or wild, while others are careful by nature. Some of it changes when they get older, but a lot stays the same. So yeah environment matters, but most parents I talk to agree with me kids come with their own character.
I expect parents expectations of how much they can shape their children has caused more broken adults than anything else.
The extrovert we teach control and restraint, the introvert to be ongoing and more expressive.
Especially parents of identical twins.
In the case of identical twins, their (biological) nature and nurture (upbringing) are the same or very similar, yet their innate character can be very different.
If you believe in the existence of a non-physical soul/spirit, that could be an explanation.
I think none of these are the same because "everything flows". With each child the environment changes significantly. The first child doesn't have any siblings to grow around, then it exerts huge influence on the second child, the third child brings in yet another dynamic. Parents also change a lot with each new child. So, except for twins, nothing is really the same.
I guess what I'm saying is that they might become different due to the environment necessarily changing in pretty significant ways for each of them.
Sadly I can no longer edit my original comment, since it's >=2hrs old, but hopefully this can stand as a self-correction.
The way parents interact with their first child and subsequent children is significantly different, and the added interaction between the children themselves compounds these.
It's still closer than comparing to a different family with different parents.
But the attention varies, youngest gets the most attention in most families.
And in many parts of the world, Sex matters too. Usually male child getting more resources.
Isn't the attention equally distributed because everybody is at one time 'youngest' ?
Ignoring changing circumstances of a family, is the environment the same?
Suppose you like sports, and get your kid a ball, but your kid likes books more, are you going to persist with the ball or get a book to read together?
If your next kid likes the ball are you going to buy more books or try more sports?
Overly simplistic but a kid is probably affecting their environment.
While I'm sure plenty does come from nurture, it's actually incredible how different they can be even before books and toys come into the picture.