is it considered drunk driving if the person in passanger seat is sober and stearing the wheel?

my wife and her friend who is kind of pregnant and that friend's sister went out for a rally that went way into the night and they had too much to drink because they were trying to warm up and on the way back the sister who was very drunk was driving becuase they didn't want a pregnant person to sit in that seat but they still let the pregnant person lean over and control where the car was moving while the drunk sister used the acccelerator and they ended up getting pulled over and the police arrested the sister was that legal ffor him to do or do they have a good case that the driver was actually the passsneger do you think?