TTC, Tests?

In our case it turned our my DH was creating antibodies that kill his sperm as they treat it as disease. My DH had a vasectomy reversal that had been successful, but this was a complication of it. So if your Sister's DH has had a reversal then I would recommend further testing even if his initial post op tests were positive.
If he hasn't had a reversal then if your Sister goes to a GP they will probably send her for a blood test to confirm that she is ovulating. If this is all o.k, then the next step would be a sperm test for DH. If that is o.k too then they will probably be told to keep trying for a longer period of time (depending on their age). If they still don't have any success they would probably then go to a fertility specialist who would do further testing for things like endo, blocked tubes, etc.