But then Phillips has never been your typical royal. From her earliest experiments in rebellion (the tongue and navel piercings), through her combustible relationship with jockey Richard Johnson, to her success as an equestrian, she has spent her life straining against the straitjacket that being 12th in line to the throne could have imposed on her.
Like William and Kate, Phillips and Tindall appeared in no hurry to get married. Although they have been together for six years, and have expressed a desire for children, they were reluctant to submit themselves to the pomp of a right royal do. Now, however, with her cousin's wedding at Westminster Abbey taking centre-stage, it seems they have spotted the opportunity to sneak in under the radar.
Despite their understated approach, however, they have not ruled out selling exclusive rights to a magazine - an offence for which both she and her brother Mark have form. He hawked his wedding to Autumn Kelly and she opened the doors of the home she shared with Johnson to Hello! magazine, in deals worth 500,000 and 125,000, respectively.
But then, that's the thing about eschewing a life of privilege. So determined was Princess Anne to ensure her children were not constrained by their royal heritage, she turned down a title and money from the civil list on their behalf.
Phillips has a royal trust fund and earns hundreds of thousands a year endorsing brands such as Land Rover and Rolex. But when you've spent 250,000 doing up your Regency townhouse in Cheltenham and it costs 500,000 a year to care for your horses, the temptation to exploit your connections for commercial gain is always there.
The image 29-year-old Phillips has cultivated of her life with Tindall as one of blissful domesticity is a far cry from her turbulent adolescence. Between gaining three A-levels at Gordonstoun and settling down, Phillips was seen as "a thorn in the Queen's side". Her name, meaning "bright as the dawn" seemed to mark her out as a maverick and she lost no time in living up (or down) to expectations. After causing a stir with her piercings, the spirited teenager spent her gap year in Australia, bungee-jumping, topless sunbathing and frolicking in the sea with fellow backpacker Angus Murray. Small beer - you might think - when compared with the toe-sucking antics of Sarah Ferguson. But worse was to come.From the moment she hooked up with Johnson, she seemed to be on a mission to self-destruct. So volatile was the relationship that they engaged in a series of public spats, culminating in fisticuffs outside a friend's home in Gloucestershire.
Back then, Phillips saw herself as something of a fashionista. The first royal to hire a stylist (Ceril Campbell), she stepped out in Vivienne Westwood, Betty Jackson and Dolce & Gabbana, often causing a stir with skirts that were either too short or split to the thigh.
Shortly after the relationship with Johnson disintegrated, Phillips was introduced to Tindall while celebrating England's success at the Rugby World Cup in Australia. He is said to have "tamed" her and helped her focus on her career as an equestrian.
Always keen to prove herself in the sport at which her mother excelled, she finally made her mark in 2005, when, together with her horse Toytown, she collected individual and team gold medals at the European Eventing Championship in Blenheim and again the following year, when she won individual gold and team silver medals at the FEI World Equestrian Games in Aachen.
Along with her mother's horse-riding skills, she has inherited her temper. In an echo of Princess Anne's infamous "naff off" outburst in 1982, she lost her temper at the Badminton Horse Trials, shouting "This is a competition, not a ****ing social event" at photographers. But she has also demonstrated an enthusiasm for charity work. Phillips is linked to Great Ormond Street Children's Hospital and Sargent Cancer Care for Children, as well as several equestrian charities.
This is not to say that life post-Tindall has been plain sailing. They have both encountered hitches in their sporting careers. Phillips had to withdraw from the Beijing Olympic Games in 2008 after Toytown was injured. He failed to make a full recovery and she claims she has never ridden as good a horse since. Earlier that year, Tindall spent three days in intensive care after he was injured while playing for England against Wales. While "recuperating", he was arrested for drink driving after a day at the Cheltenham Races with Phillips, and banned from driving. Then - to round off an annus horribilis - Phillips broke her collarbone after falling from her horse Tsunami II at a cross-country event at Pau, France. These hiccups haven't stopped her achieving, though. Earlier this year, she unveiled a new range of equestrian gear she had designed for Musto Outdoor Clothing.
A glance at the engagement photograph suggests she and Tindall are still very loved-up.But - with only 18 months until the 2012 Olympics in London, and Phillips keen to be selected for the British eventing team - it looks like children may have to be put on hold.
Perhaps the couple are hoping that - once again - Kate and William will pip them to the post because one thing's for sure, Zara "I'm not a princess" Phillips and Tindall will not want their babies to be born into a blaze of publicity, or to be seen as anything other than "commoners".