Kate isn't eligible to serve as maid of honour or a bridesmaid as those roles are reserved for single ladies. She could serve as matron of honour - essentially the same role as a maid of honour, except that the "matron" is a married woman - but according to Yahoo, it's unlikely that Kate will be in the wedding party at all.
"Kate would upstage her sister," royal expert Judy Wade explained to People last year. "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."
But with Prince George as a pageboy and Princess Charlotte as a flower girl, perhaps Pippa doesn't mind being potentially upstaged by the royals.
We'll find out for sure in five days time.
This article originally appeared on Marie Claire