The kingdom and church start inprisoning or banishing anyone who has issues with the sister, they are afraid of seeing off the miko, and since they never double checked anything are convinced it has to be her. The person here is likely the first one they exiled (personal tutor).
This will only change when the mother fall sick, as the miko's loved ones do not fall ill, and especially do not have terminal diseases (she was terminally I'll before but got better while pregnant, and relapses)
Once she falls sick the church questions the father and finds out there was a second daughter and puts him under I think house arrest, they also release the priest they had imprisoned (I think there was a real reason they should have not ignored him, but the just locked him up doubling down on their mistake) and send them to find the real miko, but do not do anything publicly.
In the middle of some political turmoil (I think when it finally came out that the sister was not the miko) the father flees, and things do not go well for him. He leaves the country, gets enslaved, runs away and tries to flee back to their village, only to get killed by monsters on the way (he has no awareness of them since the miko's blessing kept them away from the village). Only ever wanting to go back to his one daughter, never regreting how horrible he was to the one that actually mattered, shifting blame for everything that went wrong.
The sister never realized anything, and is just a spoiled brat not even noticing that the people around her are changing and others not coming back, even forgetting about having a sister. But this is a direct result of how she was raised and treated. When everything goes to hell, after it comes out she is not the miko, she actually has character growth, and starts to try and make amend for her mistakes. This is the only thing that saves her.
