This year marks 15 years in the business, a milestone celebrated when they opened Mercedes Benz Fashion Week back in May with a jaw-dropping show which effectively transported the 950-strong audience to Mars.

They’ve presented in New York, grown their accessories line to include bags, and earlier this month, as the year draws to a close, they’ve realised a dream of opening a huge new flagship store at Bondi Junction.

Sipping peppermint tea and wearing a T-shirt and her trademark dark-framed glasses, Camilla, 37, is delighted they now have a store in the Westfield they watched being built as they were kids growing up in Bellevue Hill.

“It’s extremely important to have bricks and mortar stores even when online is surging,” she says of the investment which opened last week.

“A big part of our business is customer service and to have that immediate and immersive experience with each and every customer is really important.”

This is their 11th store including online and Marc, 39, acknowledges that as well as designing clothes, they endeavour to give customers a visceral experience.

“Digital social networks are so prevalent these days which is why physical social networks are also important,” he said.

“We love to create spaces to dwell in and be inspired by.”

That Camilla and Marc are thriving in a competitive retail environment which has seen plenty of Australian designers go to the wall is a measure not just of their design smarts but their business aptitude and understanding of customers.

In person they’re cautious and measured but also tangibly excited that their little idea is now a burgeoning entity, stocked in international stores such as Neiman Marcus and desired by celebrities as diverse as Margot Robbie and the Kardashians.

For Camilla this year’s fashion week show reinforced their path:

“It took me back to our first show and it made me feel as excited as I did when we did our first one. It’s exhilarating knowing that we’re not only still here but we’re still enjoying it and feeling like we’re just getting started.”

At the heart of it all is their sibling relationship. Whether it was the death of their mum before they struck adolescence or simply a profound understanding of each other’s strengths and foibles, their connection and loyalty is palpable.

They finish each other’s sentences, champion the other constantly, live round the corner from each other in Bellevue Hill, finished high school at Moriah College in Queens Park together and laugh that, yes, yet again their families will holiday together this summer. Camilla, her husband and their three children, aged 10, eight and four, will join Marc and his wife, along with their father and his partner, skiing in America.

In April this year, a seminal moment for the brand occurred. Camilla was getting ready to go to bed when her phone buzzed with a Google alert. Thousands of kilometres away in London the soon-to-be-married Meghan Markle had stepped out with a black jacket slung across her shoulders.

Stylists across the globe would note the way the jacket was worn — slinging it across one’s shoulders was not typically regal. For the Australian designer it was a moment of pure joy: the most famous woman in the world was wearing the blazer she designed. “I was stoked,” recalls Camilla. “I phoned Marc straight away and he’d seen the news too.

“She’s a modern woman taking a modern stance in the monarchy which has very old rules. She’s incredibly relevant so it was nice and fresh to see. I got loads of phone calls from family I’d not spoken to for years and emails from all over the world.”

Marc, who runs the business side of the brand, was thrilled that Markle had chosen to wear their label — along with an Oroton cross-body bag — for the reception in April which marked Prince Harry being named the new Commonwealth Youth Ambassador.

“We had no forewarning so we were very excited and surprised,” he recalls. “She’s sophisticated and classic and exemplifies the brand.” The pair say the Duchess’s endorsement has increased both awareness and demand. So did she pay for the $699 Marguerite blazer? Both laugh. Says Camilla: “It’s quite complicated and better that we don’t comment. We felt really proud and it’s a great jacket!”

So what do they most admire about each other? Camilla, clearly the more impulsive, jumps in first: “Marc’s level-headed ability to tackle anything.”

And Marc? “We both have the ability to have true candour and honest conversations,” he says.

“We communicate really well.

“Family is one of the oldest forms of partnership and to make it work is very fortunate. We don’t take it for granted.”

Camilla was 21 and had just finished studying at the Whitehouse School of Design when she received a call the morning after showing her graduate collection asking her to showcase her designs at the “new generation show” as part of Fashion Week.

“I had a scholarship to go and do a Masters in Florence and I didn’t know how I was going to do it.

“So I picked up the phone to Marc and asked: ‘Do you want to start a business together?’” Marc, who’d been studying engineering and commerce, didn’t hesitate: “I was more interested in what she was doing study-wise and we’d always talked about doing something together.” They look back in amazement at how young they were to take on such a challenge but the drive was there.

Camilla had been creating clothes since being given a sewing machine aged 11. Indeed, she still has her first piece, “a black taffeta pencil skirt with blue zigzag topstitching”. Having been a tomboy, she suddenly started wearing dresses and by the time she turned 14 she and friends were selling handkerchief-style tops and skirts at markets.

Marc had always been interested in art and architecture and would drag his sister along to see bands.

The new Bondi store with its racks of pink and spearmint tailoring and lemon chiffon dresses showcases how far they’ve come since the days when they had to top and tail in a hotel bed in China.

As for the future, there’s been little thought for succession planning but Camilla laughs that of all her children, her four-year-old son Wolfie appears to have the keenest eye for fashion.

“He has an opinion about everything in my wardrobe!” Not that they have to take over the business. As she says: “They’ve all got such unique and different personalities and they can do whatever they want.”

CAMILLA AND MARC’S LOCAL PICKS

Best beach

Marc: Mackenzies at Tamarama.

Camilla: Milk Beach near Nielsen Park. It’s semi-private and there’s not millions of people. I love looking out to the harbour.

Best coffee

Marc: Fratelli Paradiso in Potts Point.

Camilla: I don’t drink coffee but the best tea is at Parterre Garden Cafe in Woollahra.

Favourite bar and restaurant

Marc: 10 William in Paddington and Icebergs.

Camilla: Same. 10 William and Icebergs!

Favourite drink

Marc: An organic wine.

Camilla: Vodka and soda, with fresh lime.

Signature piece of clothing

Marc: An old Dior jacket. Or a custom-made jacket.

Camilla: A blazer — our Elita jacket is my favourite at the moment.

Who would you love to have dinner with?

Marc: Camilla! Or Jimi Hendrix, the man’s a legend.

Camilla: Tilda Swinton — I love that she’s unapologetically herself.

Favourite scent

Marc: Black Tea and Wood by us.

Camilla: D.S. & Durga Coriander.