How is Rex different from the other members of his family? How does he resemble them?

Jeannette says that nothing about her grandparents resemble the man she knew as her father.

I stared at Erma and Stanley and Grandpa, searching for some feature that reminded me of Dad, but I saw none. Maybe this was one of Dad's pranks, I thought. Dad must have arranged for the weirdest people in town to pretend they were his family. In a few minutes he'd start laughing and tell us where his real parents lived, and we'd go there and a smiling woman with perfumed hair would welcome us and feed us steaming bowls of Cream of Wheat. I looked at Dad. He wasn't smiling, and he kept pulling at the skin of his neck as if he were itch