I wanted to add something about the sibling rivalry... We have DD9 who was identified *not* to be eligible for gifted services and DS7 who is Gifted (2e).

DD9 knows DS7 has skipped a grade that "he should be in 1st grade". She also knows that he can do 7th grade math and she can't.

I think no matter what, there will be sibling rivalry. DS7 wants to be "older", he often says he wishes he was born first.

I don't think (IMO), in our situation, that grade skipping and sibling rivalry have anything to do with each other. They're going to find something to be jealous of no matter what.