Madoosa, your experience matches what I have found. I find sibs harder to deal with on this issue than parents, personally, probably because of jealousy. I have learned over the years to not give the sibs too many details. For example, D has been attending Davidson THINK in the summer. When sibs or sibs-in-law ask what she is doing for the summer, we just say she is taking a couple of classes at a camp in Reno -- we skip the "college", "gifted", etc. parts of it
If they ask for details, we tell them generally what the subjects are (but "math", not "calculus", etc.). It just works out better for us. I mean, they know she is different and smart -- it is obvious as soon as she opens her mouth. But we really don't talk about it much. And she has a good relationship with her cousins, so I guess it has worked out.
