Sing out sisters

The most beautiful song ever written about love and loss is Emmylou Harris's From Boulder to Birmingham. Written just after the death of her beloved Gram Parsons, the lyric "I would rock my soul in the bosom of Abraham, I would hold my life in his saving grace. I would walk all the way from Boulder to Birmingham/If I thought I could see, I could see your face" is as sublime as how Harris sings it.

A while back, sitting beside a lake outside Stockholm, I heard two young sisters sing a song that included the chorus "I'll be your Emmylou, if you'll be my Gram" – a beautiful contemporary nod to the sad passion expressed in From Boulder to Birmingham.

The duo were First Aid Kit, Swedish siblings following in the great tradition of The Everly Brothers, The Proclaimers et al, in benefitting from the seamless harmonies and the precise vocal meshing that so distinguishes family acts.

It takes a lot of balls to do a Simon & Garfunkel cover with Paul Simon sitting just a few feet away from you. But that's what First Aid Kit did at last year's Solar Music Prize ceremony in Stockholm, which was honouring Simon. The sisters sang America so movingly, it reduced Simon to near tears and he led the standing ovation on the song's completion.

First Aid Kit give good cover version (they were first discovered on YouTube a few years ago doing a Fleet Foxes song) but it's their own material – pastoral folk undercut with deliciously embittered lyrical subject matter – that is quickly making them contenders for breakthrough band of the year.

At a White House dinner a few months ago, Samantha Cameron (weirdly, their biggest fan) spent the evening bending Barack and Michelle's ears about First Aid Kit, but it was what happened at this year's Glastonbury that sealed the deal for them. Very much on the undercard of the festival, they ended up stealing the whole show.

Figures released this week show that a good Glastonbury appearance (due to the huge TV audience) does more for a band's sales than any other festival or high-profile TV appearance. First Aid Kit's back catalogue (just the two albums to date) showed a bigger proportional sales increase than either of the other big Glasto beneficiaries, Arctic Monkeys and Kenny Rogers. And when it came to increased sales of a currently available album, First Aid Kit trounced the opposition.

There was massive media attention on The Rolling Stones, but their current album (anohter greatest hits collection) only got a 168 per cent sales increase. Mumford & Sons did slightly better, with their Babel album showing a 195 per cent increase. But sales of First Aid Kit's current release, The Lion's Roar, increased a staggering 1,000 per cent on the back of their appearance. It does reinforce the point that once people are exposed to it, there is a buoyant market out there for great new music.

It’s a remarkable outcome for an act who went into the festival known only to Samantha Cameron and a few others, and there’s some poetic justice in the fact that they beat the sales crap out of The Rolling Stones.

And the best thing about First Aid Kit is that they aren't dippy Laura Marling-style hand-wringers. Despite the gentle guitars and the soothing vocal work, there's a kick and a punch to them. Start with Emmylou and work your way up.