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.
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.
Even days after birth you can see differences. One kid settles really easily, yet their sibling would cry and need much more holding. After a few weeks, you might have a child who smiles at everybody and doesn't mind being held by others, but their sibling at that age might have avoided people they didn't recognise. etc. etc.
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.
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.