Has to be an aristocratic family? The rich and privileged members of society won't elicit much sympathy from the common movie watcher, much more preferable if it was just some random family not too interested in Syndicalism and decides to get out.
Besides, forcing them to sing propaganda songs? The communards aren't exactly Stalinist, and seems a bit too extreme in this case.
The OTL "The Sound of Music" was about the family of a wealthy retired naval officer from an aristocratic background, and most of the audience seemed to find them relatable and likeable enough to emotionally invest in the story. Also, just think of all the OTL movies and stories about members of the aristocracy and the royalty, such as.... Oh, I don't know, 1997's "Anastasia", which was about a princess escaping the execution of her family at the hands of revolutionaries? With the right story and charaterization, movie watchers from OTL clearly seem to be able to sympathise with a protagonists even if he/she belongs to a higher social-economic class compared to themselves.
As for the part about this being 'too extreme', keep in mind that:
(1) I am envisioning the movie as being produced in-universe by either a German Empire that ultimately managed to emerged victorious in the Second Weltkrieg, or a monarchist Austria-Hungary that one way or another managed to stay together as a monarchy. Both of which obviously have an incentive to demonize the communards and portray them as negatively as possible.
(2) You consider the syndies forcing a family of singers to perform propaganda song too be 'too extreme' for them? For the record, the following is the word-by-word event text for the COF if they decide to balkanize Germany after winning WK2:
'The new government in Berlin is firmly under our control and northeast Germany is now a loyal member of the Internationale. Many Junker have found their just punishment in the people's court. May their bodies, now hanging from the trees and telegraph poles, be a warning to all reactionaries!'
These are the same people that engages in acts of cultural desecration by publicly dynamiting the Brandenburg Gate while singing the Internationale, and burning down the Royal Palace of Madrid after looting the place when they take the Spanish capital. Dragging a bunch of 'reactionary class enemies' (Which the Von Trapps family most certainly count as) in front of a stage and force them to sing propaganda songs is absolutely the type of thing they would do.