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.
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.
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.