Continuing on with some of the others and trying not to mention those already listed:

The Rankin Family (family groups always get extra points)

Mac and Bob (McFarland and Gardner)~~ When The Roses Bloom Again Still gives me a thrill when I hear it

Ricky Skaggs and Tony Rice ~~ not brothers, but sound like it.

Well, Skaggs and most anybody

Buck White & The Down Home Folks (The Whites, if you prefer)

The Poe Sisters (Ruth & Nelle)