I think the key thing is that if the parent believes that a person's value doesn't stem from their IQ, but rather their character (what they do with what they have) then the kids will grow to believe it too.
Also - if the kids sense that it's a hot button issue with a parent, they'll put that button for all it's worth.
Most of us parents were children once, and many had siblings of our own. So that may be a part of life that needs 'brushing off and cleaning up' while we try to guide our own kids. In our family of 4, one kids was identified as 'gifted' and the rest supposedly weren't. I was jealous of the attention and special opportunities he got. I just was. I was also confused, because, if I wasn't gifted, then what WAS wrong with me that made me so different? The gifted-identified kid actually had much better social skills than I did as a kid. Yeesh! It's a mysterious world, that's for sure. Now I can see that talkitive and probing gifted kids have a way of getting more parental attention than their sibs because they ask the most interesting questions, and they may have more intensity pushing them to make more dramatic and energetic bids for attention.
I kind of wish I had had the chance to find out what an imperfect parent of sibs I would have been. Just like our kids, I think the best way to measure our own performance is 'Did we do the best we could with the resources we had?' We are human too, yes?
Smiles,
Grinity