My family was pretty spread out with gtness, from not gifted to pg (me). My oldest brother and sister were both hg, as was my middle sister. My middle brother and my younger brother were not identified as gt (both good artists and athletes, though), although my middle brother's daughter is hg. We don't fit the 10 IQ point average, but several of us were close together.
I tend to agree with Grinity on testing all siblings if there is a reason to test one. My older siblings talked enough and understood me enough that I did not have to show my verbal abilities until I was in school.
