Something that has helped me in a similar circumstance and it may or may not help you, but that's for you to decide. Sit with her tell her it's OK to go; that you'll be sad and miss her but that you'll be fine. She needs to hear that and it may make her passing a bit easier if she knows you will go on with your life.

It seemed to make my father's death easier for him and it certainly helped me deal with it, but I am a control freak and seeming to be somewhat in control made things seem more normal to me.

If we still lived in Houston, we could do more, but alas we moved away some years ago and this is the best I can offer.