I think that it would be...quite the upset. It depends on how wide known the circumstances of the marriage is. The sister of the emperor being forced to marry a Brettonian count is definitely something that would ruffle feathers. Maybe not outright war. But it is the kind of thing that might lead to it.
Really it would depend on how much we'd fight the marriage. If we just grimly accept it, minor fuss, marriage happens, maybe some strengthened ties between Bretonnia and the Empire. Maybe the end of the quest, though I doubt it. Worst case, if we fight it, they send an army, Franz sends an army back, and then we have a war. A bit like the Trojan War. With the possibility of vampires out of nowhere.