Dude - your comments are interesting. I know within the family dynamic DD established herself as the non academic one. She would often ask me to stop talking when DS had asked a question about something and I was explaining the answer. I am not sure about the dumbing herself down for peers, but I do think she has partly formed her school identity in contrast to DS. Unfortunately she also compares herself with him and thinks he is much 'smarter'.
Oh, yeah, lovely sibling dynamics. I didn't realize that was in play here, because that tends to play a BIG role. Knowing this now, I'd suggest that this is the primary area to focus. I assume your DS is the older one. Younger siblings tend to look for space that the older ones are not occupying as a way to compete for parental approbation. So if DS has "the smart one" role locked down, she might abandon that role and look to "the athletic one", "the musical/artistic one", "the funny one," etc.
The way I would confront this is with counter-messaging, sometimes direct, sometimes discreet. So I might directly mention DD's test scores to her, and present them as the reason I unshakably know that if she wanted to, she could do great. And I'd point out that DS is just as smart as she is, he's just been at this learning game longer. Discreetly, I'd have a conversation with my spouse within DD's earshot discussing a study which shows that second children often turn out to be smarter than their older siblings, but are often afraid to show it (whether I actually find such a study to discuss is irrelevant
).
