Why Kate Middleton won't be a bridesmaid at sister Pippa's wedding.

When the world’s most famous bridesmaid walks down the aisle in May next year, her royal sister Kate may not be by her side.

Why? Apparently it all comes down to tradition.

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

You see, tradition states that bridesmaids can only be unmarried ladies "of a marriageable age".

Considering 34-year-old Kate is very much a married woman - complete with two children - that means she's out of the running.

Kate may very well appear in Pippa's - who announced her engagement to hedge-fund manager James Matthews in July - bridal party, but she would have to be named as the "matron of honour" and drop the title of "maid".

Pippa with her sister, Kate. Image via Getty Images.

Some royal experts say that Pippa may choose to drop her sister from the festivities altogether, in fear that The Duchess of Cambridge would 'upstage' her her special day.

"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?" veteran royals author Judy Wade told People magazine.

Pippa Middleton and her fiancè, James.

"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."

Let's just hope the two sisters can work out the wedding day arrangements amicably, so they don't end up as a bridesmaid horror story in 2017.

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