Both were pupils at St Mary's when Philippa entered the Brownie contest at the Gillingham and Shaftesbury show. She was surprised that Helen, not wanting to be outdone, put her name down too – and even more surprised when Helen's cakes won the prize.
"She never cooked. I couldn't believe it. That REALLY set me off on my career," says Philippa, who now runs the award winning Mudchute Kitchen on the city farm at the Isle of Dogs in London – in that bend in the Thames you see on the credits for Eastenders.
Helen is now a well known actress and founder of Dorset Corset, the locally-based company that starts its fourth season in the autumn, taking a new adaptation of Thomas Hardy's Under the Greenwood Tree on tour of the south and west.
Competition must have been in the Davis blood – it was Philippa, not Helen, who took the leading role of Dorothy in the 2000 Shaftesbury Community Play, On the Green Rock!
But once she had settled on her chosen career path, Philippa has never looked back.
"Everything was home cooked when we were growing up, so I had a good start," she says. "We grew our own vegetables, reared our own meat and even brewed our own wine (with variable success)".
During her university studies in geography, she worked at the former Terra Firma cafe in Shaftesbury during the holidays, and she realised the academic life was not for her. So she went to London to work for the famous Smithfield butcher David Lidgate, learning from him about the selection of produce.
Then it was on to the River Cafe, where she first cooked on a wood oven. And in Ruth Rogers and Rose Gray's Thamesside restaurant she met the Sams - Sam and Samantha Clark of Moro – and was invited to join them in the kitchen of their fashionable celebrity haunt in Exmouth Market, specialising in southern Mediterranean cuisine.
Philippa worked for three years at Moro, but early in 2007 a friend told her that the cafe at a city farm in the shadow of Canary Wharf urgently needed a new direction, and she headed off to the Isle of Dogs.
She was immediately taken by the surroundings - a working farm and equestrian centre where she might return to horseriding, next to lush established allotments, and with a real family atmosphere.
But the cafe was a disaster area.
"Nothing had worked at Mudchute before," she says. "It was a chips and coke caff, not taking advantage of the produce all around it."
So Philippa from Shaftesbury agreed to take it on, and has built it up from nothing to a place where families, allotment holders, farm volunteers and city office workers come every day to enjoy simple, freshly cooked, usually seasonal delights.
The day we went it was raining cats on the Isle of Dogs, but Mudchute Kitchen was buzzing with damp and excited children, appetising smells from the kitchen and scrumptious cakes at the counter.
Philippa was getting to the end of a frantic Sunday lunch service, but eager to show of her latest acquisition - an outdoor wood-fired oven built by Mudchute volunteers led by the local vicar "a man who can find a solution to any practical problem," she says.
A short walk through the stable block brought us to the Kitchen's garden, rows of well tended herbs and vegetables in rich soil, and at the end "the smokehouse" – a tiny structure that she based on the Moro model, quite sufficient to smoke things for the Mudchute menu.
Allotment holders at Mudchute (and these are what allotments should be - community spaces with quirky personal items that include a memorial to gardeners past) are not allowed to sell their produce, but they can swap or barter.
That means Philippa and her staff and customers can take advantage of seasonal fruit and vegetables that clock up about a tenth of a food mile.
And the gardeners can swap their greens for Mudchute Kitchen food vouchers.
The atmosphere in the organically ramshackle restaurant is like a traditional eatery in Italy or Greece. The walls are lined with an eclectic selection of books. The seating ranges from re-used armchairs and sofas to tables and brightly painted high-chairs. There are toys for all ages and a rocking horse that's in constant demand.
The Mudchute association works to maintain this 32-acre green space on the Isle of Dogs, and you can walk from the cafe through the farm, the largest of London's city farms, and over Mudchute Park towards the three London Light Railway stations and the towering monoliths of Canary Wharf.
How have Philippa's innovations at the Kitchen gone down with the locals?
"Some people think it's pricey, and it is – if you compare it with places that sell burgers for £1 – but everything is home-made, and, as far as possible comes from the Mudchute Farm or from the Brooks City Farm in Leyton," she says.
The cafe has been rated as one of the top places for tea and cakes in all of London, and the lunches are equally delicious.
She and her fellow chef Louise Johnson don't lack for customers. Many of their staff are recruited from the ranks of "resting" actors, bringing an easy charm to ordering and eating at Mudchute.
"I sometimes tease Helen that I employ more actors than she does," says Philippa, so the sibling rivalry continues.
Philippa is back in Dorset this week as one of the guest chefs at Bite the Lighthouse, Screen Bites' day of food, film, music and more at Poole's Lighthouse arts centre, this Sunday, 25th October. Philippa will be on stage in the cookery theatre at about 3.30pm.
GPW