In the comments below a reader asks about the empirical difference in heights between siblings. I went looking...and I have to say that the data isn't that easy to find, people are more interested in the deeper inferences on can make from the resemblances than the descriptive first-order data itself. But here's one source I found:
These data indicate that IQ and height variation among sibling cohorts is on the order of ~2/3rd to 3/4th of the variation that one can find within the general population (my estimate of standard deviation of 2.5 inches for height below is about right, if a slight underestimate according to the latest data). But I also found a paper with more detailed statistics.
The aim of the paper was to find outliers from expectation. In other words, which siblings diverged a lot from what you'd expect in terms of normal variation within the cohort? ...
