The wedding will most likely take place in 2017 – here's what we're looking forward to...
Pippa's wedding dress
At Kate's royal wedding, Pippa managed to share the spotlight with her sister thanks to her stunning figure-hugging gown.
Although the dress was meant to be "insignificant" and "blend in with Kate's train," Pippa's outfit has been the most talked-about bridesmaid dress since.
Now it's Pippa's time to shine and we can't wait to see what the bride will wear on her big day – and the countless sketches that A-list designers will put forward.
Pippa's hair and make-up
Pippa may opt for an elegant up-do – her statement look that always suits her. She may want to keep her tresses up for ease and comfort, and bring all attention to her pretty face.
Like most brides, she'll want to look the best – and most natural – version of herself on her big day. This might mean just a slick of eyeliner, mascara and a soft shade of lipstick.
Prince William and Kate will attend
Prince William and Kate, and their children Prince George and Princess Charlotte, will add a touch of royalty to the high-society wedding when they attend.
In the past the couple have made many glamorous appearances at their family and friends' weddings, but none will be as special as Pippa's.
Kate's special role
Kate asked her sister to act as maid of honour on her big day, so it's possible that Pippa could do the same.
The Duchess of Cambridge may play the all-important role of supporting Pippa in the lead-up the wedding, and tending to the finer details at the ceremony, such as fixing her train and holding her bouquet.
Or, Pippa could turn to one of her life-long friends to take on the role, as Kate balances motherhood with royal duties. Nevertheless, the Duchess is sure to play a special part on the day that her younger sister says "I do".
Princess Charlotte's adorable role
It's likely that the wedding will take place in 2017, and if it's in May or the second half of the year, Princess Charlotte will be two years old.
Her Royal Cuteness will be the perfect age to act as a flower girl, walking down the aisle and scattering petals. Not to mention the adorable photo opportunities she'll have with the bride and her big brother Prince George.
Prince George's role
Upping the cute factor will be Prince George. The future King may act as a pageboy and walk down the aisle with his younger sister Princess Charlotte.
Or, George could be given even more responsibility and act as a ring bearer. The little Prince would carry the rings on a pillow, although the wedding planner may make sure that George is given false rings to prevent difficulties later on in the ceremony.
Michael Middleton will give his daughter away
When Kate and her father made the short journey to Westminster Abbey for the 2011 royal wedding, Michael Middleton was a source of strength and reassurance. The businessman turned to his daughter and said: "You look beautiful. It will all be amazing. You'll be fine... I'm proud of you."
The doting dad will once again play the all-important role of giving his daughter away when it's Pippa's turn to walk down the aisle.
The groom-to-be James, a "traditionalist", also asked Michael for his daughter's hand in marriage before the proposal.
Carole Middleton will be the mother-of-the-bride
Like mother, like daughter. Carole Middleton always keeps up with her daughters Kate and Pippa in the style stakes, so we can't wait to see what she'll pull out for the wedding.
The former air stewardess looked impeccable at Kate's wedding, wearing a sky blue dress and matching coat that featured braid trimming at the waist and cuff.
The outfit was designed by Catherine Walker – a favourite with Kate and other royals.
Spencer Matthews will attend
Among the high-society guests will be the groom's brother Spencer Matthews. The Made in Chelsea star may even be the best man – and deliver a memorable speech – or act as one of the ushers who will be on hand to direct guests to their seats at the ceremony, and make sure everything runs to schedule.