Kate and Will's children will also have a big part to play, with Princess Charlotte thought to be a flower girl and Prince George taking centre stage as a pageboy.
They mingled among Her Majesty's 8,000 guests who had high hopes of meeting the likes of the Duke of Edinburgh, the Duke and Duchess of Cambridge, Princess Beatrice, the Earl of Wessex, Princess Anne, the Duke of Kent and Prince and Princess Michael of Kent.
"She said that they were all really looking forward to her sister's wedding at the weekend but she was a bit anxious about how her children might behave", attendee Andrew Bates from Coventry told reporters.
The Duchess of Cambridge has revealed she is a little anxious about how her children will behave at her sister Pippa Middleton's wedding at the weekend. These reported jitters, though are not those of the blushing bride, but rather her sister, Kate Middleton. Designed by PBI Event Architecture, the handsome art nouveau structure was shipped in from Belgium and is expected to hold over 300 of Pippa and her husband-to-be's guests. According to Buckingham Palace, the Queen's staff serves more than 27,000 cups of tea and guests consume 20,000 sandwiches and 20,000 slices of cake.
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Guests have to adhere to a dress code, with men wearing suits and women required to wear a day dress, usually with a hat or fascinator.
Prince Harry will also be at the wedding, accompanied by girlfriend Meghan Markle, who is expected to attend the evening do.
This Saturday, Pippa Middleton and James Matthews will tie the knot in what seems to be one of the most hotly-anticipated weddings of the year. Earlier on in the week, a massive conservatory-style marquee was erected in the Mike and Carole Middleton's backyard. The couple will exchange vows at the quaint country church St Mark's in Englefield, Berkshire. Here's everything we know so far about Pippa's May 20 wedding.