The special "see-through metal" ultrasonic sensors are embedded in the FWD doors. The driver and passenger doors do not have any sensors. So, when you instruct the car to open the passenger/driver's door, it uses the FWD sensors to determine if there are any obstacles.
If you're wife is standing next to the FWD, then it suspects that there is an obstacle there and hence open cautiously. Ironically, if your wife stood in front of the passenger door, it would not "see" any obstacles and open right into her. I typically open the door while my wife is in front of the passenger door so that it opens completely. It's never fast or hard, so if my wife is still in the way, she can easily catch the door and stop it. But most times, she is already past the door and simply walks in.
Of course, all this is assuming that they haven't changed the design (I know they added an additional sensor in the FWD door to prevent it closing on passengers).
Also, there is an additional sensor on the roof in between the two FWDs that is used to limit how high the doors can open.