the-oldest-sibling-is-usually-the-most-well

The oldest sibling is usually the most well behaved.

1st-born children receive more parental attention in early childhood, and their main influences are generally adults. This leads to them developing better behavioral habits than their 2nd-born siblings, who get less overall attention and often learn more from other kids than they do from grown-ups.

(Source, Source 2, Source 3)