Hmmm...I've heard the 10-point thing cited often, and there's sense in it to me as a *general* rule. But I think the person is overstating the case.
Notice this in the citation:
nearly 3/4 were within 13 points.
That means that over 1/4 were more than 13 points different! So it's not likely that one sibling is GT and the other isn't, but it is definitely possible.
The big point I'd make, though, is that if the IQ score doesn't reflect what you see, then I'd be suspicious of it. Testing is one tool in your parenting arsenal, but it should never replace your critically considered mom-gut. As long as you're skeptical and not the sort to think that your child can do no wrong, then if the score seems wrong, I'd bet it probably is.
Just not because of some 10-point rule that forbids it.
