The point of miracles is that they are incredible - you have already accepted that. Saying 'incredible to whom' is therefore meaningless. You have already accepted that what floo finds incredible, you think is incredible.
No, I have not accepted that. According to the OED, incredible means:
1) Impossible to believe:
2) Difficult to believe; extraordinary:
Clearly miracles are not impossible to believe, because some people do believe they happen/have happened. Since some people believe in them they are thus not impossible to believe, unless, Floo means she finds it impossible for her to believe, in which case, so what? That was why I asked "in whose eyes" Floo thinks they are incredible.
As for the second OED meaning, again, "difficult to believe" is very subjective. Some people find it difficult to believe that animals have evolved, etc., yet neither you nor I would claim that biological evolution is in this sense "incredible".
My basic point is that Floo is saying that she doesn't believe certain things because she can't believe certain things. Again, so what?