Mmm... If I need to leave an opinion about all of this, then I'll say that there is probably a reason as to why neither the father nor the mother told anything to the MC about the true form of the curse. If a I need to guess it's probably on the lines of the same thing that Origin said before, about the MC being too nice (to the point that I even would say that she is somewhat naive for someone that was supposed to have been an adult in the past), and that it would end up with her triying to break the curse, not to help the royal bloodline, but to save the spirits with the grudge, that I assume even Origin has tried to help in the past to no aval, probably the reason being that those spirits can't really forget what was done to them and so they can't pass on. If my theory end up being true, I can somehow understand the angle taken about what to tell to the MC, and given that she is actually and adult in a child body (wich they don't know), even if they would have told her the complete story and also to no try to do anything, she would have stiked to her own moral values and would have tried to help them.
Having said all of that, I also want to point out that the curse being actually the grudge of the murdered spirits is absolutely obviously the way the author though of so that they could manage to justify having the curse removed without it necessarly being to help the royal bloodline, but actually to help those spirits.
Now for a last point, for anyone thinking that it's not the elder prince fault to having the curse explode like that, I'll say that half and half: It is not his fault for the side that it's completely obvious that the king didn't told them anything about the curse (such stupidity and selfishness...), but is also his fault for acting like he could know beter than HER FATHER about MC's health, and acting without any decorum (it was posible to do so while triying to help her) while in front of two major figures of their kingdom (the King and the Hero).