So "Johnny" is his brother? Hes older though...his birth was unaffected by Nero altering history, so shouldn't he STILL be George Samuel Kirk, Jr.? "Sam"?
Oh well. I guess there's an explanation.
Well, he'd still be
born George Samuel Kirk Jr, but
this altered timeline's young Jim Kirk grows up calling him "Johnny" for some reason, rather than "Sam". (You'll remember, in the original timeline, Jim Kirk was said to be the only person to call his brother "Sam".)
In my mother's family, several of the males, across several generations, used to call each other "Joe", even though none of them had that name, or anything like it. In my father's family, my Dad and uncle used to only call each other "Fez", dating back to an old joke between them involving the wearing of an actual fez at a dance.
Nicknames don't require onscreen explanations; they just happen. The explanations are often very underwhelming for the time it takes to explain them. Similarly, we never got an onscreen reason for "Bones" in the old timeline, only in the background material. And the new movie had fun tweaking it.