The general finding is that siblings typically post full scale scores within about 10 standard scores of each other on a deviation IQ. This doesn't mean, of course, that there won't be outliers. Most of my sibling group fall within 10 points of each other, but one person is about 50 points (estimated, since that person is well off of the standard norms) higher than the rest of us.

Another situation is if any of the siblings have significant intra-individual diversity, in which case the global measure may not fully capture their reasoning abilities. A related factor may be differences in their testability at the time of assessment (e.g., very young, with unremediated second exceptionalities, at different stages of majority culture language acquisition (if coming from a language minority background)).

Indigo also mentioned other external factors, such as ACEs (adverse childhood experiences), which may artificially lower some children's performance on cognitive measures more than other's.

So in answer to your question: Is it common? Not really. But is it significantly unusual? Not necessarily. It depends on what cutoff you use for "common" and "unusual". Not helpful, I know! But if you consider that much of the research on heritability of intelligence suggests that it is about 0.5, then it is a bit easier to see how there might be a decently-sized base rate of families with large ranges in the overall population.


...pronounced like the long vowel and first letter of the alphabet...