I asked about the sibling spread info from their webpage that samson11 quoted above. She said that those instances where they found large spreads (the 1/4 with greater than 13 pt differences as quoted above), were where one sibling had a PG IQ and another had a MG IQ -- that they hadn't found any 130/115 siblings for instance, but they had found 150/132 siblings.
Really that's like suggesting that people with a PG kid could not also have a child who was an outlier in the other direction for example a child with Down Syndrome or another developmental disability that affects intelligence. This is another strike against accuracy of GDC information as far as I'm concerned.
