I don't think the claim relies on siblings' environments being the exact same. The point is just that the degree of intra-family similarity is almost always far greater than the degree of average inter-family similarity. To a reasonable approximation it's fair to call it 'the same' in the context of the comparison with non-siblings.

I don't think that's true - the difference between being an only child and having older siblings is much more than the difference between two only children in relatively similar (from a socio-economic point of view) families, I would say.

The way parents interact with their first child and subsequent children is significantly different, and the added interaction between the children themselves compounds these.

OK, but the claim is that siblings are different, despite growing up in the same environment, because they have their "own nature and character by birth".

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.

Ah, perhaps I misread the argument actually. I agree, that's definitely an important counterpoint. Sorry about that!

Sadly I can no longer edit my original comment, since it's >=2hrs old, but hopefully this can stand as a self-correction.

> So, except for twins, nothing is really the same.

It's still closer than comparing to a different family with different parents.

Absolutely, but it seems to me that changes in the environment alone could explain the differences in character between siblings.