I have fraternal twin boys-- one is deep into PG territory and the other is HG, with much more than a 15-point spread between them. My HG son has dyslexia, which it sounds like your dd does as well. I think the dyslexia depressed his scores, as I think he is closer to his brother in many areas that the WISC reflects. But I do believe their scores pretty accurately affect their abilities-- my dyslexic son is not PG. Their sister, who does not have dyslexia, will probably have a similar IQ to my PG son, but we don't have a reason to test her yet. So, I think the idea that the scores are pretty close to each other might be true-- but probably won't be if a LD is involved.
