They can stop the labor with drugs. One of my coworkers and another coworkers wives went into pre-term labor and the birth was postponed until later - but both of them were on bedrest, so your sister may have to go on bedrest as well. Depends on how active the pre-term labor was and how long she let it go on before she got to the hospital.

My thoughts and prayers are with her - keep us updated... but if she's above 22-24 weeks, the baby has a decent chance of making it as that is when the lungs are mature enough to handle the outside world. My one coworker's wife went into labor with their triplets at 24 weeks and with the drugs they were able to hold off to 29 weeks and all 3 babies are healthy and happy and almost a year old now. No worries - medical miracles happen every day!