Here is the an article I've seen commonly referenced.
http://www.gifteddevelopment.com/articles/what-we-have-learned-about-gifted-childrenI would consider the sample rather small. They are saying 40 percent of the sample doesn't test within 10 points of each other.
I would also consider a younger sibling of a HG child may look different. My DD looked very different compared to her older brother. They have FSIQ scores 2 points apart. She is actually stronger than him in many areas, but he was so intense and she is so laid back. DD started reading at 5 compared to DS at 2 yrs old. At 7, she is the same level of reading he was at 7. It just didn't look like another PG kid when comparing her to him. My oldest DS isn't within 10 points of the younger siblings.
