Dude, omg. I just hope she doesn't really leave.
Either to force her to be a saintess/godess figure or keep her powers for themselves ヽ(`Д´)ﾉ
Maybe it's because her magic power is God's gift it is not noticed , so they think she has no magic ..
To me the "prophet" looked really suspicious... If a fight over succession could blow up this badly, there must have been fights in the background even before.
Maybe her powers got leaked and someone wants to tear apart the OP siblings and OP prince (and the probably OP not-yet-known-servant as well).
I think the reason why the prophet said she will not be the successor is because she has the healing powers. Maybe because of that, they wanted her to be some sort of godly figure rather than a retainer for the prince? This is just a random theory I had and I'm probably wrong (￣∇￣")