I read Nashville Chrome by Rick Bass this weekend. It's a novel treatment of The Browns' biography. The "chrome" in the title is a reference to their polished harmony.

Bonnie there was romantically tied to Elvis Presley before he became famous. They traveled to England to help The Beatles with their harmonies. They were produced by Chet Atkins at RCA.

I don't remember hearing them before.

UPDATE: A recent interview with Maxine Brown, with song clips.