I agree with handling the testing with tact and also reinforce the idea that though siblings are different, they may or may not have similar IQs (though different profiles). It is entirely likely that one GT kid will have GT siblings.
My brother (older) & I were both tested for school programming. We were not told our scores because at 8 & 10-- honestly the ability to wrap your mind around scores, confidence intervals, achievement vs ability, LDs, and such requires more emotional maturity than most kids that age have. Plus, the dynamic to 'compare' scores is strong at that age, easy enough to remove that temptation. We just both knew we got in to the program and the cut score was X.
That said in High School, my scores were shared with me (not my brothers and honestly we still have not compared...though I was told were are within 5 points of each other, with confidence interval taken into consideration then we have very similar final scores.). due to some LD issues and further evaluation of writing and processing. At that age, it really helped me grasp how my mind worked and helped explain the frustrations I was having.
I will say that my brother appeared classically GT to the max. Me, I was more the dreamy thinker and I always struggled with writing & speed. This was confirmed with testing that those are areas that are vastly different than my IQ- huge discrepancies.
I would do the testing and also be aware that the final # may vary a bit due to different IQ tests, confidence intervals, and such. I also would try to find a way to not share the scores as a flat #- end all be all. Scores are so much more than than---and can really be helpful to look at subscores, processing speed, and comparison to achievement testing.
