Martha Wainwright is appearing alongside a prima ballerina at Covent Garden. It's quite a culture shock

Clinging to the barre in a rehearsal room at the Royal Opera House, Martha Wainwright looks down at her feet in astonishment. The Canadian-born singer- songwriter, performing in the Royal Ballet's production of Bertolt Brecht and Kurt Weill's 1933 ballet chanté The Seven Deadly Sins, seized the black shoes of her co-star Zenaida Yanowsky at the end of the rehearsal and is now teetering perilously en pointe. "And you can actually stand in these?" she asks the beautifully leggy principal dancer with amazement.

This looks like the very embodiment of a culture clash: before her involvement in this production, Wainwright had never seen a ballet, nor had Yanowsky heard Wainwright's folk-inspired, emotionally raw music. The choreographer Will Tuckett had admired the singer's self-titled 2005 debut, however, and approached her to perform in the ballet. She plays Anna I, the pragmatic storytelling sister from Louisiana who pimps out her silent sibling Anna II (Yanowsky) as they move through seven American cities, unearthing a seedy netherworld of motel sex and exploitation. It is a bleak commentary on modern consumerism.

The new version had its debut in the spring of 2007 and Wainwright is now returning to the Royal Opera House for a second run. "Because it's Kurt Weill, it doesn't have to be sung by an opera singer," she explains. "And when Will spoke to me about singing it I said: ‘Oh my God, that would be great - can I do some movement?' Because obviously it would be a trip. When it was sung by Lotte Lenya or other people, often the singer just sat on the side of the stage, perhaps holding a microphone. But although I move through the dancers it's very important for me to stay out of the way, otherwise I'll get killed."

Just as the ballet shoes sit oddly with Wainwright's jeans, so the worlds of modern rock music and ballet seem like a strange mismatch, the formality and rigour of one clashing with the laissez-faire hedonism of the other. Wainwright smiles. "Going on stage you do think, ‘What do I have to offer? Why do I deserve to be here?' because all these people here have been working day and night, 20 years plus. And I've been at the pub a lot of the time! These are athletes of a really high standard and quality. I depend on costume."

There are, however, more similarities between their different art forms than the stereotypes might allow. Yanowsky, who gave birth only three and a half months ago, talks gravely about how many dancers go off the rails once encroaching age forces them to retire. "At 38, we are pretty much cooked and done and that's quite a young age to finish a whole career," she says. "Most of the ballerinas I have met, very few come out with their head on top of their shoulders. Most of them are slightly odd because they need that buzz - that drug - that the stage gives you. And there's nothing you can do about it because you are done, you are old, full stop."

Wainwright has also discovered that the intense physicality of the dancer's life is at odds with the ethereal cliché. "It's all about sex, isn't it?" she observes gleefully to Yanowsky, who grins wryly. "Rock musicians go out a lot, go on the road, meet a lot of people. Dancers are in here all day long and they're very youthful and kind of beautifully naive in some ways. It's like high school with a bunch of gorgeous gazelles. You see a lot of big jockstraps and bums and you're touched everywhere and the body is completely a machine."

Yanowsky laughs. "We have no boundaries, basically!"

The singer admits that the first run of The Seven Deadly Sins had been "a really steep learning curve for me". First time out, some critics wondered if her voice, even amplified, was suited to filling the Royal Opera House or fleshing out Brecht's sharp-edged satire; this time, however, Debra Craine, the Times dance critic, was among those who thought that her performance had "grown in stature".

Certainly, watching the singer stalk through the dancers' flying limbs on stage, a tough, physically confident presence in a beehive and a black dress, there is no sense that she is finding her feet. "I did what I could and sang as best as I could," she says, "but when I came back a second time I wanted to try and add some subtlety to the singing and become a bit more comfortable, a bit more of a character."

"Second times are always much more comfortable," Yanowsky agrees. "You can add more colours to the choreography; emotionally, it just grows; you dare to do new things."

Martha is not the only Wainwright to jeté across art's high-low boundaries. Her brother Rufus, best known for his baroque pop, is about to start rehearsing his long-awaited opera in Manchester, where it will open at the city's International Festival in July. He's coming to see his little sister perform on this night. "My brother Rufus has been an opera fan since he was 12 years old. That's all the music he listens to essentially and you can hear that a little bit in his pop music," Wainwright says. "But what I've heard of the opera is the real deal, it's very serious and very exciting. It's so great that both of us have been able to break into the classical world - and also not be able to escape each other." She laughs. "I'm following him wherever he goes whether he likes it or not! He's going to have to buy some ballet shoes now."

"I'll give him my pointe shoes," Yanowsky offers. "Oh," Wainwright eye-rolls, "he'll be in them in two seconds."

As the two women talk, it becomes clear that there is a happy synthesis here, fittingly, as they're playing sisters who are "essentially supposed to be the same person". Wainwright is not only the sister of Rufus but the daughter of folk singers Kate McGarrigle and Loudon Wainwright III; Yanowsky, brought up in France and Spain, comes from a family of dancers and trained with her parents.

They agree that The Seven Deadly Sins retains its impact because, as Wainwright asserts: "We are all prostitutes to a degree. Working for the dollar. It's kind of obvious - but that's OK, it's ballet!" They both laugh. "In the end you get paid to show your body and give yourself - in a nonsexual way," Yanowsky agrees.

Do you ever resent the audience as a result? "I resent a bad audience who are dead and don't clap," Wainwright shrugs. "Especially at the opera house; they are very proper people," adds Yanowsky, who spends much of the ballet in some fairly risqué underwear. "The moment you get into your knickers they are: ‘Oh my God . . .' "

"The great thing about Martha Wainwright," Yanowsky confides, "is that there is a new crowd coming to the Royal Opera House. Which is brilliant, it makes the whole audience way buzzier. It's a gamble and it's paid off well."

Victoria Segal