There's another thread on this forum where I think the mom the OP refers to might be posting
We all want our kids to be special and talented but really only time and objective assessment will establish that.
OP I'm sorry about your braggy relative, it is absolutely painful to be around someone like that - just know that your child will be far far better off than the one being raised by a mother who really seems to view her child as an accessory to establish how smart she is for her genetic contribution. It's the same logic as kids in beauty pageants.
Taking credit for a genetic anomole (intelligence, beauty, height) is far less meaningful than your child being praised for what they worked hard at (especially when it didn't come easy).
Some gifted kids have social issues, others are suffering from being raised by parents with social challenges who seem content to pass those along.