That statement about siblings usually being between 10 (or sometimes it's stated as 15) points of one another is one of the silliest things in the giftedness memosphere, IMNSHO, in that it's so often interpreted to mean "so if you have evidence from testing that this is not true of yours something MUST be wrong with the testing". Full siblings share, on average, half their DNA - the same fraction they share with each parent. It may be that siblings tend to have closer environments to one another than they do to their parents, increasing the correlation beteen siblings' IQs, but the way this statement is sometimes made you'd think there was magic attached to being siblings...
What do *you* think in the specific case of your children? Do you see differences in their thinking that seems to support the idea that they have such different IQs? If you don't, it may be that one of the tests was a freak result (or indeed that both were somewhat out, but in opposite directions).
