The Secret Sisters, Pleasance, Edinburgh

Emmylou Harris once described to me the sibling harmonies of the Louvin Brothers as sounding like they were “washed in the blood”. The voices of Laura and Lydia Rogers, two twentysomething sisters from Muscle Shoals, Alabama, have that same haunting quality. When they sing they lock together so effortlessly it’s almost impossible to discern where one ends and the other begins.

Cultural preservation is key to The Secret Sisters. Their debut album, released at the beginning of the year and executive produced by the kingmaker of US roots music, T-Bone Burnett, showcased their love of the old-time music of rural America: 1950s pop, Christian gospel, early rock and roll, classic country, traditional folk songs, even an utterly superfluous cover of Frank and Nancy Sinatra’s “Somethin’ Stupid”. Any musical innovation that had occurred since the early 1960s was simply ignored. Everything from their demure patterned dresses to their stunningly pure harmonies harked back to a simpler, more innocent age.

Singing last night accompanied by a single acoustic guitar (which they passed back and forth as they took turns strumming, as though they were sitting on a porch rather than standing on a stage), the musical backing was rudimentary, to say the least. There were moments when the thump of an upright bass or the snap of a snare would have done the energy levels no harm whatsoever, but really this was all about those voices: on the magical “Do You Love an Apple” they melted into a sublime oneness, while the closing a capella version of the old 1926 toe tapper “Tonight You Belong to Me” was genuinely jaw-dropping.

Laura, the eldest, did most of the talking, a task which seemed to come as naturally to her as singing. The topics ranged from the blindingly banal – “Yes, we are sisters, we have the same mom and dad” – to funny little asides about stalking Brandi Carlile. She embodied the spirit of the American South: waspishly amusing one minute, mawkishly over-appreciative the next. And by golly, was there a lot of talk about “daddy”, the sisters’ feelings on the subject being resoundingly closer to Jenny Agutter in The Railway Children than Sylvia Plath.

It was impossible not to be charmed as they breezed through some of country music’s great articles of faith, at all times emotionally and harmonically pitch perfect. They had recently toured with Willie Nelson so we got a tender “Blue Eyes Crying in the Rain”. They paid homage to Patsy Cline (“Leaving on Your Mind”), George Jones (“Why Baby Why”) and Hank Williams (Your Cheatin’ Heart”, “House of Gold”). It was especially pleasing to hear a fine rendition of Skeeter Davis's "Am I That Easy to Forget?"

With their (perhaps somewhat overplayed) 1940s style and home-cooked shtick, The Secret Sisters would veer dangerously close to being a tribute act - or worse, a novelty - were it not for the fact that what they do is so immaculately executed and obviously heartfelt. But what really rescues them from the danger of becoming a one-trick pony is the quality of their own material. The sweetly lovelorn “Tennessee Me” and more robust “Waste the Day” suggested they’re more than capable of adding their own contributions to the canon of old-time American classics. A new original, “Jordan River” was a rousing minor-key country gospel. Another, “Tomorrow Will be Kinder”, took that venerable roots music staple, the disaster song, and applied it to the recent tornadoes in the sisters’ home state of Alabama. The result was simple, universal and very moving.

Their hearts may be sworn to the music of the past, but as a viable creative act I suspect the future of The Secret Sisters lies in twisting the old into new shapes. On last night’s evidence they’re already doing it with some style and a whole lot of charm.

• Find The Secret Sisters on Amazon
• Download The Secret Sisters on iTunes
• Book tickets for The Secret Sisters on UK tour until 30 June

Watch The Secret Sisters perform "Do You Love an Apple"