That is what I was told about siblings as well. A significantly high percentage lay within 13 points of each other. They did say that the higher the score of one sibling the greater of a chance that number of 13% would be greater as well. But not a huge difference. I was also told, if there is a big gap between the siblings a LD is very likely, which would depress the score.
All this being said I am sure there are kids that are gifted with a sibling that is not. I am just assuming it would happen more when the gifted child is in the MG, HG range. To have a crazy, out there PG child and then one that is of average IQ with NO LD....might not happen too frequently.
