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Prince William and his fiancee also named bridesmaids and page boys for the big event, including two 3-year-olds who will be asked to be on their best behaviour during the globally televised ceremony.
The announcements by St. James' Palace made it clear the April 29 wedding at Westminster Abbey will be a family affair that emphasises the tight bonds between Britain's youthful princes and between Middleton and her younger sister, the stylish, 27-year-old Pippa Middleton, whose formal name is Philippa.
In the past, royal grooms have usually chosen more than one best man, or supporter as they are sometimes called. But the 28-year-old Prince William has made clear that Prince Harry, 26, offers all the support he needs - so both children of the late Princess Diana will have integral roles in the ceremony.
"It's unusual to have only just one man," said Joe Little, managing editor of Majesty magazine. "All of the queen's sons - Charles, Andrew, Edward - had two supporters at their wedding. But obviously William doesn't feel the need to have more than one supporter and it's by no means a hard-and-fast rule."
The recognition of Prince Harry's key role in his brother's life is one more step in the redemption of his once-tattered public image.
Harry had been derided as a carrot-topped party boy with questionable taste after he was photographed in 2005 wearing Nazi paraphernalia at a costume party, and after he was disciplined by his father, Prince Charles, for smoking marijuana. But the prince with a charming boyish smile gained a significant measure of respect recently for his military deployment in Afghanistan and his charity efforts.
Harry's on-again, off-again relationship with Chelsy Davy has also sparked speculation about a possible second royal British wedding in the not-too-distant future. The official wedding guest list - still a secret - will be closely scrutinised to see if Davy is invited, which would be widely taken as an indication that the relationship is flourishing once again.
Davy, born in Zimbabwe, has divided her time between Africa and Britain, recently leaving London to pursue a career as a lawyer in South Africa.
Since Prince Harry loves to tease his older brother, his toast to the newlyweds is expected to be one of the highlights of the celebration. Will he poke fun at William's receding hairline? Will he mention other girls who once attracted William's eye? Stay tuned.
Kate Middleton's choice of her sister Pippa as maid of honour was seen as slightly surprising because bridesmaids and maids of honour at royal weddings are typically much younger than Pippa, said royal historian Hugo Vickers.
But he said the choice makes sense because the sisters, both raised in the posh suburb of Bucklebury in horse country west of London, are so close. The younger Middleton shares some of her sister's style, including a taste for wraparound dresses, and has also played a contributing role in her parents' successful party supplies business, where Kate used to work.
Pippa has carefully stayed out of the limelight, only rarely agreeing to interviews, although she and her mother Carole Middleton were recently photographed with dressmaker Bruce Oldfield, a favourite of Princess Diana's who is seen as a contender in the royal wedding dress designer sweepstakes.
When it came to naming youthful bridesmaids and page boys - expected to provide cute photogenic fodder - Prince William seems to have exercised his royal prerogatives, Vickers said.
"Most of the bridesmaids and pages are very much from William's side, although that's not surprising," he said. "It was very much the same at Charles and Diana's (1981) wedding."
The bridesmaids include Lady Louise Windsor, 7, the daughter of The Earl and Countess of Wessex; Margarita Armstrong-Jones, 8, the daughter of Viscount and Viscountess Linley; Eliza Lopes, the 3-year-old granddaughter of Prince Charles' wife Camilla, The Duchess of Cornwall; and Grace van Cutsem, 3, a godchild of William's.
Page boys for the wedding will be William Lowther-Pinkerton, 10, the son of William's private secretary, and Tom Pettifer, 8, another one of William's godchildren.
Having the kiddie contingent so young could pose a logistical challenge familiar to any parent. Could we see tantrums as the ceremony is broadcast live to millions around the world? It's possible, but hopefully not.
"I'm sure the bridesmaids will be kept in line and not allowed to run amok," Little said.