I hope everything goes well and I wish your sister all the best.
That said, this is her pregnancy and her baby. You and your mom don't get a vote. It's one thing to talk it through with other people but to gather opinions and information to try to sway her/guilt her/scare her into doing what you want is over-stepping boundaries.
It'll be the same after she has the baby. Her baby, her decisions.
I would have been LIVID if my mother, friends, or siblings thought they got a vote in my pregnancy and birth.