Knitterly, it's not the exact same situation, but I kind of know where you are coming from.
From the time he was an infant, my son showed evidence of keen intelligence, so it was no big shock to us when he was, at age five, identified as G&T and placed in the G&T cluster at school. My family was delighted, and I shared the news with my MIL, thinking that my son's other grandmother would be as happy and proud, too.
Not so. She has always favored my STBX's siter's kids, and this was no exception. She denigrated and negated our news, saying it was no big deal, that Russ was nothing special, and that we ought to keep him out of G&T so he would not feel "superior" to other children. And anyway, my SIL's kids, (all of whom has special needs and/or struggled in school), were every bit as smart as Russ. I was just like, Ooooo-kaaaay, whatever, and inwardly rolled my eyes. I mean, it wasn't meant to be a *contest*; I was just excited sharing news about my kid/her grandson, not making any comparisons whatsoever. SIL's kids have their own unique abilities and personalities, and being identified as G&T doesn't make my kid "superior", just his own person. From that point on, we never mentioned anything about his achievements to her unless asked directly, which was seldom. He has never warmed to her, and they have a rather distant relationship, (although I know they love each other.)
My advice to you is just do what I did. Let them ask, don't volunteer anything, and allow LK's achievements to speak for themselves. The validation you are looking for might not be forthcoming, but in the meantime, you are the mama of this sweet, beautiful, amazing little girl whose milestones will continue to amaze and delight you for...well, forever! Mine is 15, almost 16, and doing college-level work. We are at the point now of waiting to see what he decides to concentrate on once he gets to Uni, and seeing him into the adult world. The fun really never ends; I enjoy Russ more the older he gets.