I will definitely keep your whole family in my prayers. As someone who spent weeks 20-24 upside down on Magnesuim Sulfate (nasty drug but does the trick) I can definitely emphathize.
Wow, down to 23 weeks now, huh? 9 years ago DD was just a viability at 24 weeks.
If your sister does deliver early -my DD is 9 and solid A,B student w/ not lastig effects and that was even 9 years ago. They can do miracles.