FJ that's a tough one isn't it?! It's so hard trying to convince someone like that (When I say 'like that' I don't mean that in a bad way- hope that doesn't come across wrong!).

I don't have any articles but I can say that I am a tiny person and birthed my girl no worries. She was posterior too. I was so worried early on that being so small would mean that I'd run into troubles but I had the opposite. Completely natural, no pain relief (just had a few stiches). I guess the best thing was that after DD was born I was up, had a shower and felt great (tired but honestly felt really good)just about straight away. Now how different is that compared to having a c/s?! i could enjoy my beautiful bub, and concentrate on bf.

Prob not much help, but I do hope you find a way to get through to your sister, and hope she changes her mind!