Deb's answer makes sense to me...
My son wasn't at his sister's birth because at barely 2 we felt that it would't really be practical, both in terms of preparing him and in terms of me being distracted by his presence. I would imagine that with a nine-year-old it could be a very special experience for all concerned, providing that you are comfortable with it as well (If you would rather not, than tell her so and you can always work out a compromise such as having her somewhere nearby and allowing her to see you immediately after the birth).