Here is another post on this issue, which quotes the IAS3 on this facet.

Note that IAS3 acknowledges awareness of positive anecdotes regarding skipping a child into the grade of an older sibling.

IAS3 also discusses skipping a child into the grade of an older sibling as an issue of "family dynamics". Family dynamics are within the family's control and parents may seek assistance, possibly from the psychologist, for support with managing family dynamics, including any issues of sibling rivalry.

Here is a quote from Hoagies Gifted Education Page:
Quote
There are a few critical issues that make a whole-grade acceleration not recommended. I'm not sure I agree, but... they drew the line somewhere. IQ less than 115, no problem there. Student is against the acceleration, again I agree. The next two are the ones I have trouble with: sibling in the same grade, or sibling in the receiving grade. According to the scales, these are critical items, and totally negate the idea of whole-grade acceleration. I disagree there - in my opinion we should always treat our kids as individuals, and though a lot more care needs to be taken, these two issues should not make whole-grade acceleration a total "NO".
But that's pretty much the only thing I disagree with.