Six years ago on the edge of a wood in Stockholm, teenage sisters Johanna and Klara Söderberg sat down with a video camera and recorded a cover of one of their favourite bands – First Aid Kit were introduced to the world.

Today their rendition of Fleet Foxes’ Tiger Mountain Peasant Song has nearly four million views, twice as many as the original, and the Swedish siblings, now aged just 23 and 21, are one of the most exciting folk acts around.

“It was so amazing. We just went out into the forest outside of our house and shot it for fun with a simple digital camera, one take. We didn’t think a single person would ever watch it.

“Shortly after we uploaded it on Youtube we sent it to the band and the next day they wrote back to us saying they loved it.

“We were completely overjoyed. The video got so many views after that. We’ll forever be grateful for it, the song really did a lot for us.”

Fans of the pair have seen them grow from grinning teens with bobbed hair to towering, vintage clad Nordic princesses. The obvious constant is in their music – which has always been about baffling, hair-raising harmonies and catchy country melodies.

It’s a style that is certainly aided by the closeness of the relationship between the two.

“It’s definitely much harder to sing with someone else. You don’t know how they phrase things so you always have to be on your guard.”

“We barely have to think at all when we harmonise with each other, it’s so natural to us. Whatever we come up with harmony-wise the first time we write a song is usually what we end up going with on the album too.

“For us it’s probably the easiest part of our job.”

On the cusp of releasing their third album, First Aid Kit now stand as one of the many bright lights of Swedish music, alongside the likes of Robyn and Lykke Li.

The sisters kicked off the tour for their new record Stay Gold at Islington Assembly Hall last night and are now heading across the Atlantic for a month of shows in the states.

“We’ve had some of our best gigs in London. We’ve heard from others that London crowds are tough but our experience has definitely been the opposite – we feel so much love here.

“Last summer we played Somerset House and it was magical. We hope the new songs go over well with the audience.”

Stay Gold is released on June 10.