Its easy to make up just-so stories to explain differences across siblings as being caused by birth-order.  This article casts doubt on the significance of birth order.

But we can ask the question of whether birth order should matter and in what ways.  Should natural selection imply systematic differences between older and younger siblings?  Here is one argument that it should.  Siblings “share genes” and as a consequence siblings have an evolutionary incentive to help each other.  Birth order creates an asymmetry in the ways that different siblings can help each other.  In particular, oldest siblings learn things first.  They are the first to experiment with different survival strategies.  The results of these experiments benefit all of the younger siblings.  (Am I a good hunter?  If so, my siblings are likely to be good hunters too.) Younger siblings have less to offer their older siblings on this dimension.

As a result we should expect older siblings to be more experimental than their younger siblings and more experimental than only children.

Here is evidence that older siblings have more years of education than younger siblings and more years of education than only children.