Found this interesting info on the motivation of the IAS as it relates to skipping a child into an older child's grade:
http://www.springville.k12.ia.us/vnews/display.v/ART/51856c345f5efThe IAS is adamant that if the student in question has siblings in the same grade or grade above that other forms of acceleration be considered. By altering the grade of a sibling in close proximity to another, the family dynamic could be altered as well.
Your counterargument is sound: the family dynamic is not altered, therefore, this concern is not valid.
To help as parents, we practiced ageism at home (when we hadn't before). We allowed the older one to stay up longer and have other privileges.
To offer some contrary anecdata, I have a brother two years older whose physical and emotional maturity level was always quite close to my own (he was delayed due to infant medical issues). I never saw him as an older brother, but as an equal (and sometimes as an inferior) peer. If my mother had practiced ageism between us, I would have punished them both.
