"We did something we never thought we’d do today," announces Klara Soderberg to the tiny backroom at New York’s Mercury Lounge, so crammed full of people that they spill out the doors to the bar. “We sang backing vocals for Lykke Li on The Late Show with David Letterman!” There are a few laughs in the audience, a few impressed gasps. “It was a really tiny studio. You should feel sorry for us,” Klara’s older sister Johanna deadpans.

Save your sympathy – First Aid Kit’s story is the stuff of dreams. Just three years ago the Swedish sisters played their first London show to a handful of people who’d discovered their cover of Fleet Foxes on YouTube. Now that same video has two and a half million views. By the time they played the last show of their first US tour in New York last year – after releasing an E.P. called ‘Drunken Trees’ and a debut album, ‘The Big Black and Blue’ – Karen O and Nick Zinner of the Yeah Yeah Yeahs came out to see them and the sisters were so happy they cried.

Tonight First Aid Kit headline the poky Mercury Lounge ahead of an opening slot for fellow Swede Lykke Li in New Jersey. Demand for tickets must have been high, as it’s announced shortly before the show that First Aid Kit will be back in the States in March, headlining the 1,400-capacity Webster Hall. That’s a huge leap – recent headline shows at Webster Hall include Laura Marling and St. Vincent – but if tonight is anything to go by, First Aid Kit might just pull it off.

Video cameras roll during their performance, capturing Johanna’s grins when she stumbles on her harmonies, and Klara’s headbanging during ‘I Met Up With The King’, her long hair flailing as she rocks out on an acoustic. They step in front of the microphones for an unplugged rendition of ‘Ghost Town’, which quickly becomes a sing-a-long. Everyone knows the words. The sisters smile encouragingly.

First Aid Kit specialise in textures – their voices switching from angelic to cracked and raw on old favourites ‘Our Own Pretty Ways’ and ‘Tangerine’.

The full force of it isn’t apparent until a new track called ‘EmmyLou’. When the percussion cuts out for the penultimate verse, Klara’s voice suddenly turns soft and clear as a bell. “I’ll be your EmmyLou and I’ll be your June, you’ll be my Gram and my Johnny too,” she sings in homage to some of the musicians whose influences breathe through everything First Aid Kit write, and when she’s done, and the drums come back in, the whole room emits a long sigh.

The Lion's Roar, comes out on January 24 via Wichita Recordings.

By Hazel Sheffield. Blogs at The View From Here and tweets at @hazelsheffield

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