Family at the birth?

Naomi
My sister and I are a few years apart and I'd say we are pretty close, usually. However she has been really strange since I let her know we had conceived. She is angry with me that I didn't consider her schedule in the timing...
The week of the due date of my first child is also the week of her 30th birthday. She's let me know that even if she misses the birth she will be throwing herself an extravagant birthday (on the other side of the country). And she really resents me because "it was so important to me to be there for the birth, and now I can't be and it's your fault!"
What do I do? I'm really befuddled by her anger.