Can you maybe sit with her and help her write out a birth plan?
Ask her what she would or would not like to concentrate on this time round, ask her what she wants you to do while there.
Does she want you to be an advocate and fight for her birth plan, does she want you to just be there to comfort her and hold her hands?
I think asking what she wants from you is a great place to start, gently discuss if she has any fears and how you might help her work through them.
what a wonderful sister you are.