Here's why Kate Middleton won't be sister Pippa's bridesmaid.

It’s the thing every bride dreads: being upstaged at your very own wedding.

It can be argued that Pippa Middleton did exactly that at her sister Kate’s wedding to Prince William in 2011 in that bridesmaid dress, and now that the tables have turned, all eyes are on what Princess Kate is going to do.

Pippa Middleton at the wedding of her sister Kate to Prince William in 2011. Photo: Getty Images

The Duchess of Cambridge is said to be "absolutely delighted" at the news her younger sister is engaged to hedge-fund manager James Matthews.

But will she want to risk potentially overshadowing her sister at her own wedding? You know, there's that whole 'future Queen of England' thing to consider... Accord to veteran royals author Judy Wade, Kate won't take the risk.

Pippa Middleton and her fiancè, James Matthews.

"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?" she tells People magazine. "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." Comparisons between the two sisters have already begun, with commentators focusing on the size, shape and quality of their engagement rings. Yes, it's come to this.

Pippa's ring is said to be between 3-4 carats, while Kate's - which once belonged to Princess Diana - is a 12-carat sapphire surrounded by a halo of 14 diamonds.

At the end of the day, a ring is a ring, love is love and a wedding is just a wedding. We're sure the sisters will figure out the arrangements for themselves - bridesmaid or no, we're sure Kate will be supporting her little sis no matter what.

Want more of Pippa? Take a look through our gallery...