Color Doppler is the u/s technique that can diagnose vasa previa.
Vasa previa is extremely dangerous; I've just been diagnosed with it myself. The fetal mortality rate is astoundingly high, 50-95%, when undiagnosed. The problem is that when the mother's water breaks, the unprotected blood vessels running through the amniotic sac will also rupture, and the baby will bleed out within minutes.
The recommended protocol is hospitalization in the third trimester (in case of PROM, so an emergency c-section can be performed; you have only minutes to get the baby out safely, so the time from home to the ER can mean you're already too late) and a scheduled c-section by 35 weeks. Steroids to help mature the baby's lungs can also be helpful, and an amniocentesis can help determine if the baby's lungs are mature enough for delivery.
It truly and totally sucks, and I've been alternately in tears and enraged since I found out on Friday, but the danger should not be underestimated, and your sister shouldn't let anyone tell her it's less of a big deal than it is. The real problem would be if your sister's OB WASN'T pushing for a c-section. There is no other safe way to deliver the baby. (Believe me, I HATE this piece of information, but it's true.) The good news is this tragedy can be prevented: the survival rate for babies when VP is diagnosed and the protocol is followed is 97%. It's a rare condition, and even rarer that a u/s tech even looks for it, so your sister can count herself lucky (as can I; our u/s tech is getting a present when this is all over for literally saving our baby's life).
There are several research papers available on the Internet as well as a very good informative site where your sister can start:www.vasaprevia.com
Good luck to your sister!