I think it is important to note that this sort of jostling for position happens in EVERY sibling relationship, GT or no, accelerated or no.
I think it is possible to worry too much about that sibling relationship or attribute too much of what trouble occurs to acceleration, when some of that would be going on anyway, even if everyone in the family were ND.
As an older sibling, I guess I don't feel like the older sibling's position should be sacrosanct. As Cathy notes, little siblings have to wrestle with insecurities every day. I don't think it's necessarily bad for an older sibling to have to deal with being less good at something, too.
Of course, that's easy for me to say, since it's not likely to be a problem we'll ever experience...
