While I do blame the royal family for being shits in general, I can't actually blame them for forgetting about the cause of the curse.
Spirits are long-lived compared to humans, and the royal family's already been through several generations since it happened. To them it's not only an inordinate amount of time, but from the moment they're born, they have what they feel to be an unfair burden saddled on them. There's a fuck somewhere in that lineage that failed to pass the knowledge along, but that's not the descendents' fault either.
Contrast with the spirits, most of whom were probably around when it actually happened, and it's no wonder it seems like the family hasn't learned a thing.
I can't say I'm too sympathetic towards the spirits regarding this marriage fiasco though. The goddess of marriage sorta lost any justification with me the moment it was revealed she could separate marriages and yet did nothing to keep the princess from forcibly marrying into the family.