I haven't heard why the Holmgrens stopped after Zonhoven. Obviously Ava crashed, but I can't imagine their whole plan was to do only 2 races each during the kerstperiode. Both are now down in Spain training like everyone else, but neither is signed up for Benidorm.
Bella had been doing some rides with some of the Canadian CXers and her dad, and today posted a ride from the Lidl-Trek road team camp with Ellen van Dijk and Riejanne Markus among others. Ava doesn't appear to post on Strava, but seems like she's done at least a few easy rides with her dad, also rode with her dad in Roubaix before heading down to Spain. (Mr. Holmgren is a cycling coach for many of the younger Canadian 'crossers)
Gunnar Holmgren appears to be taking part in a Trek MTB team camp, also within a stone's throw of Benidorm, but won't be racing this weekend.
At this point one can only hope the siblings Holmgren are still planning to take part at Worlds, because the sisters should be locks for a top 10 finish in the U23 race, in addition to real podium possibilites for Raphaelle Carrier (juniors) and Ian Ackert (U23). Gunnar Holmgren is no favourite for a top 10, but the likeliest man for a decent finish should be selected to race.