Swedish sister act First Aid Kit is polite enough to acknowledge its celebrated YouTube cover of Fleet Foxes’ “Tiger Mountain Peasant Song”, but it’s easy to understand why the pair would rather talk about other things. For one, the video, which features flannelled-up siblings Klara and Johanna Sí¶derberg cooing together in an idyllic outdoor setting, was posted nearly two years ago. More importantly, the duo’s recently released The Big Black and the Blue is full of folksy originals that eclipse that early homage. The YouTube clip did, however, introduce millions to the outfit’s awesome vocal chemistry, which the siblings chalk up to their sisterly bond.

“I find it really hard to harmonize with people other than Klara,” Johanna admits, on the line with her sister from their Stockholm abode. “This is so easy; it requires no thought at all. She could sing anything right now and I’d harmonize to her in an instant.”

“It’s kind of like harmonizing with yourself in a way, because our voices are so similar,” Klara pipes in.

While the acoustic-guitar-toting Klara initially started First Aid Kit as a solo act, Johanna quickly signed on as covocalist, leading to the group’s first release, the duet-heavy Drunken Trees.

Unsurprisingly, The Big Black and the Blue focuses on the siblings’ intertwined vocals. Opener “In the Morning” starts out with gentle six-string, drops off into a cappella territory for a minute or so, and then unleashes shimmering shaker beats and fingerpicked guitars before returning to hauntingly spare singing.

“I think that the focus will always be on our vocals and the lyrics of our songs,” Johanna says. “We don’t want to drown them out with too many instruments. I feel like people sort of expect us to have a richer sound, but sometimes I feel it’s unnecessary.”

That’s not to say The Big Black and the Blue is devoid of elegant instrumentation. “I Met Up With the King” breezes by on smoky campfire guitars and sing-along vocals, but the addition of Johanna’s autoharp and an airy Mellotron elevate it all to something grander. Despite the song’s heaven-sent sound, the lyrics—highlighting the struggles of a disgraced king, a lovelorn lady, and Klara herself—are a bit of a downer. The final verse finds her comparing herself to the monarch and alluding to some sort of betrayal, although she never quite explains what’s going on.

“It’s about people not being taken seriously, outsiders who don’t get as much respect as they deserve,” Klara says before cautiously backing off from a full explanation. “I don’t want to dig too deep into it.”

As generally tightlipped as the sisters are about their lyrics, they will allow that much of The Big Black and the Blue—from the jug-band shuffle of “Sailor Song” to the rail-riding ballad “Ghost Town”—is obsessed with travel. With its busy tour schedule, First Aid Kit is looking forward to finding plenty of new inspiration.

“I think it’s going to influence our songwriting,” Johanna suggests. “We’re going to sing about it even more. Our second album is going to be called First Aid Kit on the Road!”

Though Johanna cracks up at the thought of the title, her sister is hardly amused. Despite their musical connection, Klara is quick to point out that she doesn’t have the same sense of humour as her sibling.

“No,” she deadpans. “That would be horrible.”

First Aid Kit plays the Media Club on Sunday (June 6).