Planning a flight near due date

So we are buying my sister's plane ticket so she can come up when the babies are born. She can only stay for a week and since it's twins I feel like it's harder to pin down a date. My c-section is scheduled for August 30, when I'll be 38 weeks. At first she suggested that she come up the 16th bc no one thinks I'll make it to 38 weeks, but I brought up the fact that if she comes then she may miss their birth and have to go back before they're even born. So now we're thinking the 27th, the Saturday before. This way if I deliver on the 30th she'll be here for the delivery, she'll fly back out in 8 days on September 4th. So will only have 2 days with us at home. If we make it to 38 weeks. Sigh, such a dilemma! It was so much easier when we lived in the same state. The longer we wait, the more expensive it is and I'm not sure if there will be a cost to change the date. Has anyone planned something like this? Or have any other ideas?