... doesn't every family need a few colorful characters in it's history?
I haven't found any of those often rumored Diehl / James Gang connections yet, but --
There's a chap named Charles Ray "Pony" Diehl / Deal that appears to be more than just legend, reportedly a former member of the John Kinney Gang, Diehl joined the Jesse Evans Gang during New Mexico's Lincoln County War. Afterwards, he went to Arizona and joined the Clanton "Red Sasch" Cow-boys. He was sought after by Wyatt Earp during the Earp Vendetta Ride but wasn't found. Some claim he killed Johnny O'Rourke in 1882, after O'Rourke allegedly killed his friend and Earp enemy, Johnny Ringo. There is even a Wikipedia entry for "Pony Diehl" as well as any number of other internet sightings -- just Google for them.
Now, if you like to play with those seven degrees of Kevin Bacon games, and it seems that most of us do just that, intentionally or not, well, Pony Deal was a friend of Johnny Ringo (John Peters Ringo, b May 3, 1850), and Ringo was a boyhood neighbor & contemporary of the James boys, and a possible cousin of Cole Younger.
Does anybody have any real family history on this Charles Diehl?