there is a bit more to it then that. if I remember right the parents were the shitty ones in this situation and did not let others know about the second kid and took their favorite to the temple because of course she was the miko.
someone gave a breakdown so I don't know how accurate this is.
kids typically cant use magic so there is no real way to test her
there is no simple magic finding spell, just general directions, on top of not knowing if you can even really test the mikos power
I don't think the parents and that sibling 'trick' them for long, but it is long enough that finding the real one is hard, this one only got found because of a hunch.
At least up to the point they described, the humans aren't morons, but they have no way to know or verify along with the parents sending the real one out to die.