Also, Bob and Ron Copper aren't brothers but cousins...But to say they could be Americans from their accents? No way!

Other possibilities that spring to mind: The Millen Family (well, they're from Kent, so no closer)

The Watersons (Yorkshire, but mostly sisters...and revivalists to boot, and certainly not on records fifty years ago!)

No other sibling groups come to mind for me....

Nerd