But will she be in the bridal party as matron of honor?
The elder sister, 34, faces an age-old royal conundrum of not wanting to cause an upset by having her princess status overshadow the bride, 32, on her big day.
And Kate will want to do her best to avoid that.
“Kate would upstage her sister,” says veteran royals author Judy Wade. “It’s a tricky situation for Pippa. She would want her sister by her side, as who else would she trust to make things go well?
“But if your sister is a future Queen, she is going to upstage the bride and draw attention. Kate would want to be in the background as much as possible.”
One pointer from the past: Princess Diana wasn’t a bridesmaid at the prominent wedding that followed hers – the 1986 nuptials of Sarah Ferguson to Prince Andrew. Her own older sisters, Lady Jane Fellowes and Lady Sarah McCorquodale, were both already wed by the time Diana became a royal.
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With her own two children Prince George and Princess Charlotte – who may have their own wedding roles to play – Kate will be busy ensuring they don’t dominate proceedings by dashing around the church.
5 Things to Know About Pippa’s Fianc James Matthews!
There’s every chance that Kate could do a reading – just as the sisters’ brother, Boomf marshmallow maker James Middleton, did at William and Kate’s wedding in 2011. James could well repeat that role and may even be best man – he has taken part in grueling athletic events alongside Matthews and Pippa and also has vacationed with them – or certainly be an usher.