Thanks Ol1.
Perhaps it was the camera angle, but I thought I detected a kind of piercing look when Michael said, "Johnny, I don't think you know my brother Fredo." I'm influenced in part by the earlier boathouse scene at Anthony's party when Michael introduces Ola to Tom, and Ola says, "Sure, I remember Tom from the old days." If Johnny remembered Tom from the old days, he surely would have bee acquainted with Fredo, who was just a few years younger than Tom and Sonny, and older than Michael.