"Prince George (who is 3) will be a page boy, and Princess Charlotte (2 on May 2) will be a bridesmaid", the statement added.
A spokesman for Kensington Palace said: "The service will be attended by close family and friends, including The Duke and Duchess of Cambridge and Prince Harry".
At the Royal wedding in 2011, Miss Middleton was maid of honour when she stole the spotlight from her sister with her figure-hugging striking white Alexander McQueen dress.
Pippa and James became engaged in July past year, when James got down on one knee in Britain's breath-taking Lake District and presented her with a diamond ring worth somewhere in the region of $300,000.
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Britain s Prince George gets up on tiptoes to peek into the pram of Princess Charlotte, flanked by his parents Prince William and Kate the Duchess of Cambridge, George will be a page boy and Charlotte a bridesmaid at the nuptials of Pippa Middleton, right, and financier James Matthews. Spencer will likely serve as his brother's Best Man during the ceremony.
When Prince William made headlines for his Swiss ski getaway, it turned out that Kate was on her own holiday in the Alps, although her trip was a little more under the radar.
About 150 guests are expected to attend, according to The Mirror, although some estimates expect 350 attendees. "If your sister is a future Queen, she is going to upstage the bride and draw attention". Kate could, however, still have a presence in the service by giving a reading. But pitching her look perfectly for her sister's wedding is set to be one of her most fascinating sartorial challenges yet.
Pippa Middleton is getting married in May (Photo: ISP / Splash News).