Graeme Thomson

Musicians are often prone to measuring success in terms of symbolic journeys. For Prefab Sprout it was Langley Park to Memphis. For Aztec Camera it was Westwood - in East Kilbride - to Hollywood.

The last time Swedish sisters Klara and Johanna Söderberg spoke to the Herald, it was early 2010 and the pair were just about to play Sneaky Pete's, a rather bijou, unpretentious room in the belly of Edinburgh's Cowgate. Five years later the pair are heading for the ornate splendour of the capital's Usher Hall. It's not a long walk on foot, but in career terms it's a huge leap. Maybe one day they'll get a song out of it.

Back in 2010 First Aid Kit had just recorded their first album of harmony-washed folk-rock, The Big Black and the Blue, and released it via a tiny indie label. Now, they're touring their third album, Stay Gold, bankrolled by Sony and a top 30 hit in the UK and the US. The finer details of Edinburgh's live music scene may be lost on them, but the sisters are well aware of how far they've come. "I don't think then we thought we could make a living out of it, but now we can and we just want to continue on that path," says Klara, the younger and more talkative of the pair. "We're determined to make the music we want to make."

The daughters of Benkt Söderberg, former guitarist in one of Sweden's biggest rock bands, Lolita Pop, the Söderberg sisters grew up surrounded by music. It wasn't until they fell for indie-folk band Bright Eyes in their early teens, however, that they started to unearth the music which would go on to inform their own records. "Bright Eyes and the film O Brother, Where Art Thou were our starting points into folk and country," says Klara. "So thanks, Coen Brothers! Then we went back and explored the sources. We both love Carter Family, which is the beginning of the whole thing. We've been obsessed with that, and it's just been so fascinating and so much fun to go back and explore."

From those beginnings, First Aid Kit discovered other formative influences, such as Emmylou Harris, Gram Parsons, Townes Van Zandt, Lee Hazelwood, Neko Case and Fleet Foxes. Indeed, with their Summer of Love vibes, flowing robes and flower-strewn hair - not to mention the Americanised twang of their perfectly spoken English - there's a sense that they're almost too eager to pay homage to their sources. The word 'derivative' has occasionally been thrown about. How do they plead?

"I don't think it's bad to wear your influences on your sleeves," says Klara. "I think it's good to hear where something came from."

Johanna interrupts, "As long as it's not a copy!"

"It's good if our music reminds you of something, if it doesn't sound exactly like it," Klara continues. "We're still exploring now, but when you're younger it's even more obvious. Especially on our first EP, I can hear when we're trying to sound like someone else. There's a charm to that, I think it's quite lovely. I never mind if someone tells us we sound a bit like Neko Case or whoever. That's never going to bother us. The hope is that people will hear those names and think, 'Who's that?', and go and listen. That's a great gift to give someone."

Stay Gold, in any case, is a clear progression. The sisters' dazzling harmonies and sun-kissed blend of past and present Americana is fleshed out by a 13-piece orchestra, lending the album an epic, dramatic feel. If the music soars, however, the lyrics often brood, reflecting the preoccupations of two young women who spend most of their lives in transit, away from home and those they most care about.

"It's not something we planned," says Klara. "We wrote these songs about our lives, so I guess it's not so strange that those are the themes, because that's what we've been doing. Travelling a lot and being away from people we love, going on these great adventures."

Johanna mentions that many of the songs are about "being lost, in a way. We're pretty happy, but we're very emotional. We can be very up and also very sad."

"I think the way we deal with that sadness is that we write about it," says Klara. "Our music is our own first aid kit. Without sounding too cheesy, that's really the whole idea behind the name. We help ourselves and hopefully by helping ourselves we can help others."

"I think with the record we're growing up and realising that things aren't as easy as you think, and you're not the person you thought you might be," says Johanna. "You make mistakes but you just have to go on because it's worth it in the end. If you don't get the hard stuff you don't get the good stuff."

It helps that the band is a family affair. Their father left the rock and roll front lines many years ago, but he remains their road manager, while the tightness of their harmonies is the most obvious manifestation of a close-knit sisterly bond. "We trust each other, it's really good," says Klara. "What we do is sort of organic. We've been singing so much in the past seven years, it's comes to us naturally. Usually, the first harmony that pops into my head is the one we end up working with and recording, so it's a pretty natural process. We never really have to think about it that much." Getting Sneaky Pete's to scan might just require a little more graft.

First Aid Kit play Usher Hall, Edinburgh, on Monday. Stay Gold is out now on Sony