Pinker starts his chapter on children with the so-called three laws of behavioral genetics:
The First Law: All human behavioral traits are heritable.
The Second Law: The effect of being raised in the same family is smaller than the effect of the genes.
The Third Law: A substantial portion of the variation in complex human behavioral traits is not accounted for by the effects of genes or families. The laws are about what make us what
Pinker, Steven. The Blank Slate: The Modern Denial of Human Nature (p. 373). Penguin Publishing Group. Kindle Edition.
The pillar of the Second Law is the evidence showing that siblings reared in the same family are at most only slightly more similar than siblings raised apart. He concludes from this that parenting choices have very small effects on children's development.
It seems to me that he is leaving out a very crucial aspect of the unique environment of a child reared with a sibling. There is only so much social/emotional ecospace in a family, and that fact induces siblings to choose different paths. If the first child is a hell-raiser, the second may become more docile to fill the empty spot in the family ecosystem, and vice-versa. Similar effects can occur for the math whiz, sports star, science geek, etc. If this difference in environment is as large as the effects of different parents it helps explain the fairly large Third Law effect and tends to discredit Pinker's version of the Second Law.