Does dead average mean her FSIQ is right at 100 or it's up around the higher end of "average" on the bell curve? Were her subtest scores consistently "average" or did they vary wildly? If your dd is struggling with reading issues, and if the subtest scores show variation, it's a good idea to look at them in detail to determine if visual processing or other challenges might have impacted some of the subtest scores.
I am sure there are genetically related siblings who's IQ varies by more than 1 SD from each other - my guesstimate is based on knowing quite a few families who have one child who qualifies for our district's HG magnet program and sibs who don't qualify (IQ is one of the bars that has to be met to get into the program). I've also known a few families where their children were admitted into the program in Kindergarten, the kids didn't perform amazingly so they were retested a few years later and were taken out of the program (by the district) because the new ability testing came in lower than their 5-6 year old testing. That doesn't mean your 6 year old dd's testing isn't accurate - it just means it's *possible* it's inflated. One of my dds had a HG score on the WPSSI at 5 years old but has been tested twice since (WISC-IV and WJ-III Cog) and on both of those tests, at a later age, she is clearly about 20 points below where her early testing put her - which, for her, fits better with what my gut feeling has always been and with how she performs in school.
So.... my suggestion is, pay attention to your gut feeling. For your older dd, take a good look at her subtest scores. Don't focus on whether or not sibs should be close in ability range, just focus on each child, does their individual data make sense, do you need more/better data etc.
Best wishes,
polarbear
