Charles has 2 engagements on 12 Dec, so I wonder if he will cancel them to attend. He and Anne are the only members of the British RF to have engagements listed on the BRF website for the 12th.
Queen Maxima has an engagement on the 12 according to the DRF website, so does the King of Spain.
The King, Queen and Crown Princess of Sweden as well as Prince Daniel have a meeting with a Government minister listed but I guess this could easily be moved to a later time/date.
The Danish Royals have no public engagements listed neither do the Norwegian Royals. The Grand Ducal website has nothing listed for Henri or his family but this might not be up to date.
I guess Albert could fly over to Belgium for the service at least and then fly straight back, I know in the past he has flown with the Belgian Royals to/from Royal events so they must be quite close, it depends how Charlene is feeling at the time I imagine.
With all those mentioned they may very well psotpone/cancel their planned engagements. I've always had the feeling Fabiola (and Baudouin) were well respected and well thought of by other Royals so I imagine many of them will try to attend.