Maybe Fred is getting some sort of treatment in South East Asia. Getting over the shock of the twins, and his various peccadillos. That is usually a sound explanation for a family member missing a big family occasion. And yes, the royals are a single extended family IMO.
Interesting Hester; I had not considered this. And you know, you would think we'd be seeing some kind of pictures of him at the convention, and even of the Dame at his. Maybe the Dame is keeping an eye on Fred while he's getting treatment, and neither are really at their respective meetings.