I'm not a parent, so I can't give you much advice there. However, I am a younger sibling. And when I was in 6th grade in the late 1970s, my sister was in 8th, and we were both tested for the gifted program in our district. She didn't make the cut. In fact, the school psych reportedly told our mom that my sister was likely to drop out of high school.
On the other hand, he thought the 5th & 6th grade enrichment class would not be stimulating enough for me. I ended up in the 8th grade enrichment class--with my sister's classmates--but still in the sixth grade classes for the vast majority of the instructional week. But that's a whole other story.
I can't remember my sister saying anything about this situation, and I can't imagine what she could have said. I did move into adolescence with the understanding that I was not allowed to make any friends in my sister's class or the class in between our years!
My sister did not drop out of high school. The school psych was her instructor in the community college, and neither one of them ever mentioned his prediction. The same sister who did not make the cut for gifted class is now a medical doctor in charge of the training of radiology residents. She makes somewhere between ten and twenty times as much money as I do. I try not to talk to her about money. I seem to have some 6th grade karma to work off, because I now teach 6th grade!
In this day and age--by Renzulli's Three-Ring criteria--my sister's task commitment and other attributes would probably land her in the gifted class. Those attributes, an educational experience that had her in or near her zone of proximal development most of the time, and a sense of having something to prove are probably big factors in her success.