Theodora, who is also known as Teddy, will join Prince George and Princess Charlotte as she performs her starring role at Eugenie and Jack Brooksbank's wedding at 11am at St George's Chapel, Windsor Castle.
But this royal wedding brings up a "genuine debate" about Britain's royal family, said BBC News' Simon Bates.
"Expect headline-grabbing gossip that'll leave palaces squirming as we get to know the real people behind the titles", Katy Manley, creative director at Initial, the United Kingdom production company, said in an interview with Variety in July.
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According to Parker, Eugenie's ring wasn't meant to be a copy of Catherine's ring, which originally belonged to Princess Diana.
Rounding out the bridal party are six-year-olds Maud Windsor, whose parents are Lord and Lady Frederick Windsor, and Teddy Williams, daughter of singer Robbie Williams and Ayda Field.
The unexpected drop-in followed Samantha appearing on a British talk show to publicly apologize and plead with Meghan for a meeting to hash out their issues and reconcile.
Williams has been a friend of Eugenie and her elder sister Beatrice for several years, after meeting their mother Sarah, Duchess of York, on holiday in the Caribbean.
Guests will also be asked "to surrender mobile telephones, cameras, and any electronic devices used for image capture" to the reception following the ceremony, which will be held on Friday.
Eugenie is ninth in line to the throne.
The princess and her fiancé invited over 800 people to attend their wedding and guests such as Kate Middleton, Meghan Markle and The Queen are all expected to attend.