John Stillwell/PA/Landov
Simon Perry
May 18, 2012 10:00 AM

The Duchess of Cambridge was looking good for Her Majesty on Friday.

She joined husband William and his brother Prince Harry for a special lunch at Windsor Castle during which Queen Elizabeth entertained monarchs from 26 countries and most members of her immediate family.

Kate, 30, wore a $2,000 coat dress designed by Emilia Wickstead. Described as a pleated dress with a square neck, it is made of double crepe wool and is from the current collection.

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The stylish Kate was seen chatting with William, 29, and Harry, 27, in the Waterloo Chamber at the Castle before a lunch of English asparagus tartlet, new season Windsor Lamb with braised potatoes and vegetables – and a tomato and basil salad.

Dessert was strawberries and vanilla charlotte.

Guests included Prince Albert and his wife Charlene; the former king of Greece; the kings and queens of Denmark, Norway and Sweden; and King Abdullah and Queen Rania of Jordan.

Among the younger British royals: Princesses Beatrice and Eugenie, who attended with their father Prince Andrew. Later, Prince Charles and his wife Camilla are hosting a dinner for the visiting sovereigns back in London at Buckingham Palace.

For more on Kate’s sexy style, pick up the latest issue of PEOPLE, on newsstands now

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