Is the wife sinful if she goes against the doctor's advice to have a natural birth instead of a C-section, even though all indicators suggest there is no danger to her or the fetus?

It is the doctors who decide how the birth should proceed, and they do not resort to a C-section unless it is absolutely necessary. We advise leaving the matter to them. If you are in doubt, consult another trustworthy doctor. If they confirm the necessity of a C-section, then comply. If they mention harm to you or the fetus in a natural birth, then you must implement their advice, for warding off harm is obligatory, as the Prophet, peace and blessings be upon him, said: "There should be no harm nor reciprocating harm."