Kate and William were at a Lincolnshire church to share the joy of his cousin Emily McCorquodale and her groom James Hutt, whom she met on a blind date.
The Prince and his brother Harry grew up playing with the bride, who is the daughter of one of Princess Diana's elder sisters, Sarah, and her husband Neil McCorquodale, a farmer and former officer with the Coldstream Guards.
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According to the Telegraph, Emily and her siblings George and Celia used to holiday with their aunt and royal cousins on Necker Island, Sir Richard Branson's Caribbean island retreat.
Guests in the picturesque village of Stoke Rochford on Saturday included Earl Spencer, Diana's brother, and his heavily pregnant third wife, Karen.
The couple were joined by Lady Kitty, one of four children from the Earl's first marriage.
Also with them were the son and daughter he had by his second wife, eight-year-old Edmund and Lara, six, who acted as a bridesmaid.
The Duchess – who was a picture of demure sophistication in a dove grey ensemble, most of which had been seen before –made sure not to steal the limelight on Emily's big day.
Derby in 2011 and the floral Jenny Packham dress made an appearance on her tour of North America.
Meanwhile, the coat was recycled from the Knights of the Garter ceremony last year.
The wedding was an especially joyous occasion since nearly a decade ago Emily fought her way back from cancer.
While studying at Leeds Metropolitan University, she received radiotherapy and chemotherapy to treat plasmacytoma, a potentially fatal cancer of the plasma cells.
The Princes have remained close to Emily, who was invited to the royal wedding with her family.