based on my experience, when i tried to apply (hospital/clinic) they usually asked me if I am NHA certified, means you have took the exam.. but for hamad, you apply first then they'l schedule you for an exam. Good luck to your sister!

by the way, just for your information, its a bit hard if not impossible to look for a job here if under personal visit visa. that was my problem. they prefer under husband or father-sponsored visa. just pray~