First Aid Kit - Royal Albert Hall, London 24/09/14

Tonight, all that glittered was gold. From her long hair to her shimmering shoes, Johanna is golden head to toe, with Abba-esque shiny trousers, high waisted and flared. Her younger sister Klara is in an iridescent dress, also gold. As they perform, they jump up and down like little girls at a birthday party wearing their new sparkly shoes.

First is Stay Gold, the new record out this summer, alongside the support of Stockholm Strings who are given a flattering introduction. Klara's gutsy and intricate voice engulfs the auditorium. At the keys, Johanna playfully stomps her feet, often playing one-handed with the other gesticulating in the air. Their voices really are exceedingly good, with harmonies that play tag and perform trills like little songbirds from Snow White.

"This is bloody marvellous" cries a plummy voice from the back of the regal hall when the last note silences, as if entering a competition for the most British thing to ever happen. The sisters humbly blush and express gratitude. It is all jolly lovely and la-de-da, spiffing and tally ho and what not.

The direction of their thanks then swings to their family, many of whom were watching in the audience, and a song is dedicated to their sound engineer dad whose been at every gig they have ever done. It was all so sweet I nearly contracted diabetes. These guys make the Brady Bunch look like the Mitchell brothers.

The songs are beautiful yet soporific and with little bite. When they cover Jack White's “Love Interruption”, the venom inherent in the track is only barely tangible. The two of them flick their hair as if head-banging and stalk the stage with an attempt at attitude - in fact they are about as rockstar as Rick Astley. But fair play to them for mixing it up a little and injecting some energy into an otherwise soothing hot bubble bath of a gig.

It's pretty, real pretty, but it does begin to waft by in a floaty wishy washy fuzz. Then Klara announces: "we wanted to do a song without microphones.". A quick survey of the hall reminds me that this is really quite a large venue, albeit known for its acoustics.

As the two inch to the front of the stage, the nerves are palpable. They stand like children about to do a spelling bee. Klara gets her hair caught in her guitar strap, and Johanna has to untangle it. They seem so utterly vulnerable that we wait with baited breath, willing this to be a success. And then they begin, and their voices true and clear ring around the hall.

Having invited the audience to sing-along, a few brave souls begin to softly whisper and a gentle communal murmur builds, though it suddenly teeters out as the girls hit an excruciatingly high note. I don't think many of us could strike that one - nor, does it seem, do many of us know all of the words to “Ghost Town”. It’s a shame as the audience were keen to contribute - we just needed an easier brief!

Blue light cascades upon the stage, leaving our two heroines shimmering like golden mermaids. They exchange a few words to their "offensively handsome" drummer, and a multi-instrumentalist from Brighton who has been ramping up a number of tracks with his electrifying dexterity. Then they close on “The Lion's Roar” with a drum rolling, spotlight swirling finish.

What followed can only be described as rapturous applause; sustained raucous hand-clapping that pleads insistently and persistently for an encore. Which we got. "Play “Emmylou!”" was demanded as the girls wandered back on stage, which was rebuffed by a giggle and a promise to do so.

With “A Long Time Ago” they’re back in their comfort zone, and this is where they excel. It’s beautiful, and they are at their most captivating when deep felt visceral emotion. The cover of Simon & Garfunkel’s “America” that follows has vocal patterns that are faultless.

A ripple of laughter precedes “Master Pretender” as the single cover apparently bore a warning for having "explicit content". The thought of anything but pink bows adoring their singles is quite frankly ludicrous.

As expected, they close on an extended version of “Emmylou”. "Let’s sing together", they invite, and we do. It is a special moment - if a little marred by a rather enthusiastic though absolutely tone-deaf singer who obnoxiously hawks from the back of the room, instigating another little titter. To end on such a splendid song and a shared smiling moment is enchanting.

Sugar and spice and all things nice – that’s what these two girls are made of. But behind their butter won't melt image are fierce vocals and courageous pleated harmonies. With melodies I was still humming as I wandered home.

• Photo by Sara Amroussi-Gilissen. See the full gallery here.