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.