Hi FionaJill,
Sorry for butting in here. I really am not sure what to tell your sister but I think the article you are after is: Caesarean Myths Exploded. It is nice and easy to read and matter of fact not totally freak out material.
Also I read somewhere about the pygmies who are tiny (der, obviously) but who average 8lb babies and all naturally! So "small pelvis, schmall pelvis!!!!" The body of both the mother and the baby are designed to work together or we would have died out as a species hundreds of years ago!
Hope that helps your battle a bit!
Sazz