Wort, one of Luxembourg's daily newspapers, is reporting that the Duke and Duchess of Cambridge will be attending the wedding of the Hereditary Grand Duke and Countess Stéphanie de Lannoy.
I am skeptical of this report for several reasons. I do not believe that the guest list has been set in stone. The Duke and Duchess of Cambridge may be among those who receive invitations but it is the ultimate decision to send a representative will the Queen. In 1981, the Duke of Edinburgh represented the Queen at the wedding of Guillaume's parents.
Guillaume is the heir to Luxembourg throne. William is second in line to the British throne. Guillaume did not attend William's wedding. His parents attend William's wedding. It is more likely that the Queen will send the Prince of Wales to represent her.
On another level, Guillaume's wedding will be a good opportunity for William and Catherine to meet many of their European cousins. The presence of the Cambridges will also mean more press because the British and most of the European press will be more interested in what Kate is wearing than in the bride!
However, the British never confirm attendance at foreign royal events until largely the last minute so we won't know who will be attending from the British royal family until a few days before Guillaume's wedding in October.
but Guillaume is the heir to the throne.