I didn't have the 12 scan with either of my boys, but was told at my 20 week scan with DS2 that I had 3 soft markers and 1 hard marker (short humerous). Together these gave me a risk of 1/20. I had a second opinion scan a week later and they found only 1 of the soft markers (which was very mild) and the humerous length was normal. My risk was back out to 1/600. DS2 was born without any problems.

Please tell your sister not to worry until she has had the second opinion scan, easier said than done, I know. I assume that if she is coming all the way to Brisbane then she will be going into QLD Ultrasound for Women? They are fantastic in there... spend ages doing the scan and they have genetic counsellors who speak to you about your risks and what they mean etc.

As the other posters have said, the NT measurement is not very reliable at 20 weeks. They are called soft markers for a reason, and if that is her only marker (you didn't mention any others), then it probably means nothing.