It was a couple of years ago when I first noticed that the fashion pack had developed a ridiculous habit of banging on about how normal they are. It coincided with the release of The Devil Wears Prada. Apparently, Meryl Streep’s spot-on portrayal of a catty career gorgon hit a nerve, because they’ve been crying foul ever since. Talk about self-delusional. In truth, most of the fashion lot are totally nuts — childish, bitchy, shallow as rock pools — and none more so than the tricksy designers. This turns meeting Christopher Kane into a nice surprise.

He is the country’s most exciting womenswear talent right now. By the time he graduated from Central Saint Martins in 2006, Donatella Versace had already taken note — even comparing him to Gianni — and offered him a full-time job, which he turned down. But now, he’s designing accessories for Versus, her diffusion line, with ready-to-wear to follow shortly. Add to that the new collection he has produced for Topshop — the store’s biggest designer collaboration to date.

Scanning a rail of his new range, I’m compelled to make the lame-but-necessary inquiry: “What was your inspiration?” Kane, gesturing to a punky little fluoro number, mutters in his sweet, nasal brogue about a picture of a Moroccan belly-dancer in a mirrored top he once saw, but soon runs out of steam and emits a hefty sigh. “Look, it’s a really nice dress, and hopefully it will sell,” he says, cocking an eyebrow. “There’s my inspiration.”

His success is extraordinary, but Kane is, as he would say, “dead normal”. Thrillingly so. He likes Loose Women, tea and dogs. He doesn’t snort coke off Zaha Hadid coffee tables or dip-dye his bichon frisé to match his loafers. He doesn’t even have a bichon frisé. Instead, he has Tammy, his 31-year-old sister, who is infinitely preferable. Five years his senior, she is involved in every aspect of the business, working on the designs as well as being his muse. She rejects this last label, mostly because it doesn’t do her justice. She has been crucial in turning the hyped graduate from daydreamer to icon in three years flat. These days, he is stocked in 150 shops around the globe and is the darling of Carine Roitfeld, the editor of French Vogue, with women queuing up to squeeze themselves into one of his outrageously sexy, if dangerously snug, creations.

So I fetched up to meet the pair at their studio in east London, to see what life is like after you’re the next big thing. Clearly, they have come a long way. It’s an achingly cool 4,000 sq ft warehouse with 10 staff and a few visiting bods from Topshop. “A bit House of Eliott, isn’t it?” Christopher laughs. He is impish and charming, glancing shyly out of the corners of his eyes when he says something wry, which is often. Tammy enters a few moments later, hair scraped back in a do that, on another woman, I’d call a Croydon facelift. Somehow, it only adds to her authoritative and, I must admit, rather sexy vibe.

The pair are not fazed by the hip surroundings. Why should they? Christopher has worked with the likes of Kylie Minogue and Beth Ditto, and his collections routinely sell out within 24 hours of hitting the shops. He says he doesn’t even do much designing here, preferring to slob out at home in front of the box. “Television’s brilliant,” he says. “It relaxes you, doesn’t it?” agrees his sister.

“We grew up on it,” he continues. “It’s all we did: Sons and Daughters, Neighbours, Prisoner: Cell Block H. And we still do.” It’s hard not to titter at the idea of Roitfeld in a creation dreamt up to the Neighbours theme tune.

They continue to natter away, much as they have since Christopher learnt to talk. By that time, the pair’s older siblings — a sister and two brothers — had left home, so Christopher and Tammy spent most of their childhood at the family home in Motherwell. Their father, who died a few years ago, ran an engineering company, while mum raised the kids. The two were inseparable, often sharing a bed, but destiny yanked them in a new direction the moment Christopher picked up a pen. “When he first started to draw, I was mesmerised,” says Tammy. “He’s been able to draw like a fine artist since he was six. He won Scottish national competitions.”

“Oh, stop!” he butts in.

“But it’s true,” she says, “and nobody knows about it.” She turns to face me. “I had this thing that I was obsessed with him, what he was going to do next. Then the fashion thing came along.”

“It was The Clothes Show,” he laughs. “Before that, I didn’t know what being a fashion designer was.”

Tammy studied at the Scottish College of Textiles, before Christopher breezed through his interview for Central Saint Martins and became the star of his year. Versace admired him so much that she donated the fabric for his degree show. He had an instinctive feel for how to make clothes that grabbed attention but that also — gasp! — women might actually want to wear.

I wonder if Tammy resents toiling in his shadow? “That’s a horrible question to ask!” she cries. “We have no issues, never have. We’ve not been brought up like that. There is not one thing I wouldn’t do for him.” Do they fight? “We play-fight,” he says.

“We have differences of opinion,” she adds. But they’ve never stayed angry for more than half a day. In fact, they do everything together, even holidaying with their boyfriends. (He’s been seeing Declan, a lawyer, for four years; she Richie, an engineer, for seven.) There is tension brewing now, though, as Tammy wants to bring in a designer to help with the workload — common practice for any fashion house. “I know he doesn’t want it,” says Tammy. “He hates anything that’s not his.”

“I’ll just ignore him,” says Christopher, flicking his hair in mock annoyance. They start to laugh, scraping their chairs back before heading back to their desks. It occurs to me that perhaps you can be in fashion and normal after all. The Kanes are the exception that proves the rule.

Christopher Kane for Topshop is available from Sept 18

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