Hi FJ,
Sometimes it's easy to make a judgement about someone elses birthing decisions, but really it is up to your sister. If she has genuine concerns and has discussed these worries with her health-care providers, then the decision is up to her.
I don't think trying to persuade her to your way of thinking or scaring her into a vaginal birth is the answer.
Sorry to throw a spanner in the works, but she is entitled to give birth however she pleases. Perhaps the very best thing you can do for her is to support her, regardless of how you feel about how she brings her child into the world.
LJ