I'm not sure if this is exactly the same thing but my sister gave birth to twins 9 weeks ago and she was told at a scan 1 week before they were born that there was a problem with the bowel of one of the girls. She saw a consultant and had one of those 3D scans. It showed that part of the intestine was blocked or not developed - they couldn't tell which. Anyway, when the twins were born, little Beth had an operation less than 24 hours later to fix the problem - basically what they did was cut out the affected section and sew the two ends together. As it happens the initial operation didn't work properly and she had a second one about 10 days ago but she is now on the mend and doing well.
Tell your sister not to worry too much. These doctors do know what they're talking about and the fact that they've spotted it before the baby is born is really good as it means they'll be prepared for anything that needs to be done. When the twins were born they had 2 pediatricians in the delivery room and as soon as Beth emerged they took her for xrays etc and took really good care of her.
Give my best wishes to your sister and tell to try not to worry too much - if the baby needs an operation it will be hard to cope - he/she's likely to be the neo-natal ward for a while but my sister's experience is that the nurses in those wards are brilliant - really caring and knowledgeable.
Hope you can put her mind at rest a little and let us know how she gets on thursday.