Last week, Kate and her husband Prince William were invited to dinner at Pippa and James' home.
Express.co.uk has contacted a representative of Pippa Middleton for comment.
From the sound of things, Spencer will be attending his brother James' wedding solo, as Pippa doesn't think it's in good taste for Vogue, who's a reality star, to be mixing it up with royalty - especially when they haven't been together all that long.
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The bride-to-be has declared a strict 'NO RING, NO BRING' policy, meaning Spencer will have to attend without Vogue.
Some say it's because Vogue's relationship with Pippa's future brother-in-law, Spencer Matthews, is so new. The following year she launched a junior brand to the firm called First Birthdays, and continued to work with her family until January 2011, just months before she married William. Kate wowed as she hit the red carpet in the red tea-length tulle gown, prompting the label to be "flooded with sales requests" from fans hoping to recreate her look.
She's also anxious because their relationship is still in the early stages and only wants plus ones to be married or engaged to the invited guests. "It's certainly raised eyebrows". Pippa is now preparing for her wedding and her sister is there to help her be on top of everything.
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Rumours have been swirling surrounding Pippa's big day, as she prepares to tie the knot with James this spring, with the details so far kept under wraps.
Pippa and James will Wednesday at St. Mark's Church in Englefield, near the Middleton family home in Bucklebury, England. It's not yet clear what role Kate, the Duchess of Cambridge, will have in the wedding party of her only sister.
In fact, many fans throught Pippa overshadowed Kate at her own wedding. The reportedly $200,000 ring features a stunning Asscher cut diamond.
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With regards to the after party of the wedding, Carole and Michael's Georgian Grade II-listed manor home will be the venue.