the miko brings divine benefits, but also divine punishment. The reason the boot anyone sis does not get along with is that they are afraid of upsetting her and bringing that punishment down on themselves, they are certain that sis is the miko because they used a divine oracle to locate the miko, i believe the priests fell comatose afterwards so it can not be easily re-done, and sis was the only one the priests found that matched the age.
The nation as a whole entered an age of prosperity with Lerunda's birth, which is why they checked for the existence of a miko, and they know the miko's displeasure would end that (i believe it is already gone, and the village getting the backlash worse), but I think when I skimmed some chapters it said that nations have been destroyed for displeasing/disrespecting the miko. It is kind of gard to nantain a bation if you suddenly gave no crops.
Also from that I got the "enemy" nation ends up in a slave revolt.