The IAS is based on research, and through the research it was determined that when one child accelerates into the same grade as an older sibling, statistically speaking, the skip is more likely to cause more problems than it solves. Does this mean a grade skip into a grade where an older sibling is present won't work? No, it just means that according to the research behind the IAS it is more likely to cause more problems than it solves. The IAS specifically recommends NO SKIP in this scenario and suggests instead the next best thing- subject acceleration. I only know this because I've recently combed through it.
