The sold-out audience talked back to sisters Johanna and Klara Soderberg in Swedish accents, which caused the siblings, better known collectively as First Aid Kit, to blush and laugh. The duo, who are in their early 20s, stepped onto the stage of the Crescent Ballroom to a crowd that knew them better than they knew themselves.

This created such an ease between performer and listener that no calls for audience participation were necessary when the band began with "Shattered and Hollow," a song from their upcoming album Stay Gold. The Soderberg sisters were almost thrown off from their two-part harmony to accommodate the singing from the members of the audience. The crowd even kept rhythm with their hand claps during "King of the World" from their previous album The Lion's Roar.

If people are paying this much attention to a Swedish band that is releasing its major label debut in less than two weeks, then it was easy to tell who was late to the party. You simply just had to look around and find who is just standing around. Those people somehow managed to go six years unaware of the country-folk coming from the land of the Vikings and Steig Larsson novels. It wasn't difficult for the uninitiated to be charmed by the band, dressed in shiny gold dresses and eager to please.

Klara Soderberg's vocals were strong and rapturous, resembling former Rilo Kiley lead singer Jenny Lewis, as her blonde haired sister sang Johanna back up. At one point, an audience member caught Johanna's attention and requested they play their version of the Fleet Foxes song "Tiger Mountain Peasant Song." which Klara admitted they hadn't played in four years. Occasionally they would have to ask the audience for the next lyric but they nearly pulled it off. The duo, free of vanity, smiled their way through the song to appease the audience member's request and won more respect from them in the process.

Years ago, it was a video of their rendition of that song that went viral and First Aid Kit has never looked back since. There is something a little off-kilter when you realize that two 20-something Swedes have been singing about small town waitresses ("Waitress Song," off the new album) and finding themselves while traveling across the country. It's not that there's a rule that only Americans can sing Americana, but it's hard to tell if the sisters are even qualified to have had the experiences they are singing about. Neither of then have graduated high school. Their father, who is also their sound guy, home-schooled them. Does the songwriting even come from an honest place?

That question was answered when First Aid Kit started with a cover of Simon and Garfunkel's "America" to start their encore. The girls are obviously not singing about their own exploits, but their idea of what country and folk music is to them (and possibly should be) from listening to it. There is something romantic about that music, with its tales of hitchhiking across the country and meeting vagabonds. Singing about that romanticism is just as honest as experiencing it yourself. The point was further driven home when they closed with their hit "Emmylou," a homage to country musicians Emmylou Harris, Johnny Cash, and Gram Parsons. When Johanna and Klara sing that they'll be my Emmylou and June, we all want to be their Gram and Johnny too.

Critic's Notebook

Last Night: First Aid Kit and Willy Mason at the Crescent Ballroom

Personal Bias: Four years ago I went to see a little band I had never heard of called Mumford and Sons and everyone knew who they were but me. I felt that way this evening.

The crowd: A variety of vintage shirts, pixie haircuts, and thick rimmed glasses and the parents supervising those who wore them

Overheard in the Crowd: "Carson Palmer just has to learn his routes and he'll be fine" -- the three middle-aged gentlemen behind me who don't think it's too soon to talk about football.

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