William and Harry
The two princes are older than Beatrice and Eugenie, but they have always got on well with their cousins. As William was getting more serious with Kate, and Harry was splitting with Chelsy Davy, the younger prince used to party with his cousins in London, thanks to their overlapping friendship groups. They even vacationed together in the Swiss ski resort of Verbier on New Year's eve in 2011.
Eugenie gave a reading at her cousin Peter Phillips' wedding in 2008, while William and Harry read at Zara Tindall's wedding in 2011 — so could the royal brothers be on hand for similar duties on Eugenie's big day?