R.R.
I would plan her birthday party for the week before, and I would talk to her about how her new sister will be like a birthday present for her that she can keep always! :-)
My Mom went into labor on my 6th birthday, so we all went to the hospital and my third brother was born the next day! He was two weeks early. So ever since I refer to him as the best birthday present ever even though I was hopping he was girl. :-)
Just keep doing what you can to help her be happy and feel special, and see how it all plays out!