When my sil was at the hospital in labor w/ my nephew, I talked w/ my brother. He was very stressed out about his wife having surgery, he was driving home to get clothes, he needed someone to talk to on the drive. (I have no idea if he remembers it this way or not.) He also needed to focus on the road, so I think we talked a bit and then hung up.
They knew I was a VERY vocal homebirth/no drugs/no circ advocate (we went around on the circ issue a few times) yet their birth did not go that way.
My 20 yr old sil was in SURGERY. My little brother (we're 6 yrs apart) was very scared for his wife. They needed love and support and hugs and for ppl to be positive.
I didn't bring up natural birth, what they did 'wrong', what they should have done, what they needed to do next, or rant about hospital births and interventions and doctors.
You still have time (I hope) to talk to your sis about her options. Suggest that she try walking, try this, try that, whatever comes to mind that will help her get the best birth she can. But know that you can't change it. Be supportive as best as you can.
After Simon's birth my sil went on a self-exploration journey, so to speak. She went from thinking she was broken to knowing she isn't. It's taken her 4 years of healing but she's done it. Yes, she had a c/s, but she was able to use it as a learning experience. If she had ppl around her telling her how awful it was form the get-go maybe she'd have had a harder time, maybe she wouldn't be the UBAC queen that she is now.
You love your sister. Be there for her and listen when she talks. This will be her journey and where she goes after is up to her. She's gonna need someone like you to be there and hear her and support her.
I hope she has a really, really good birth. Let us know how she does.