Dog on Dog Aggression

I have two Jack russell terriers. The male is 3 years old, and the female is 2 years old. They are from the same family, but one litter apart. My questions is to figure out why when someone comes to the door who is not a member of my family(mailman, friends, neighbors) does the female become so aggressive with the male to the point where she latches on to him and we have to physically separate the two so she doesn't cause any serious harm to him. At no other time does she do this, only when someone is knocking at the door, or waiting to come in. once inside, she is fine and fine with others.