Looks like they're the sons of [one of] the Good Brothers. See
http://www.nowtoronto.com/issues/17/51/Ent/cover.html
"...Long before the battling Gallaghers ever considered the publicity potential of regular dustups, the musical benefits of brotherly competition that the Kinks, Beach Boys, Sonics and Stooges explored could be heard decades earlier in the magically intuitive interplay between Bill and Charlie Monroe, Alton and Rabon Delmore, and Ralph and Carter Stanley.
That lineage carries right through to Dallas and Travis Good, the roughhousing guitar duo behind the Sadies' twangy malevolence. They understand implicitly their connection to the great tradition of brother musicians. It's actually encoded in their DNA.
As the whip-smart sons of Bruce Good -- who along with brothers Larry and Brian Good form celebrated Canuck country hell-raisers the Good Brothers -- Dallas and Travis were born to wear fancifully embroidered suits, strum mandolins and dobros, sing in close harmony and settle arguments with their knuckles."