It sounds like she is resource guarding and protecting your wife as a valuable resource that belongs to her.
This means your dog has a couple of wrong ideas to work on.
One is that the dog believes she is in charge and running things. This, in doggy social situations, gives her the right to bite to emphasize her corrections of bad behavior.
The other error in her thinking is that acting like this gets her more of your wife's attention.
The way to correct this is to 'demote' her from queen of the universe (or at least the household) and to reduce the attention she gets from your wife when she acts out on the principle that acting that way gets you less of what you want and to get more of what you want you have to behave differently.
For the demotion of her or elevation of the humans in the house I suggest the nothing in life is free program which you can read about here
http://www.paws.org/cas/resources/fact_sheets_dogs/nothingfree.php
To changer he mind about others approaching your wife - the dog should not be allowed up on the furniture with her and training sessions should be started where when someone walks in the room the dog gets petted and when they leave the petting and attention all stops. What this does is change the viewpoint of the dog about the results of others coming near your wife by making good things happen for the dog when others come to visit.
Hope this helps you!
Dog Expert:Rescue, Train,Breed,Care
30+ yrs dog home vet care & nursing, rescue, behavior&training, responsible show breeding, genetics