This is a territorial behavior that should have already been corrected by the dog's owner(s) a long time ago. Unless the dog's owners are willing to work on properly training the dog, the behavior will likely continue. The dog has come to believe that the home is HIS space, not the space of his owners. If the dog respected the home as the territory of his OWNERS, he would not behave this way. The dog does not view you as a threat. If the dog viewed you as a threat, you would see lip curling, growling, snarling, and snapping. The dog views you as an INTRUDER, and a threat and intruder are not necessarily one in the same as far as a dog is concerned, so trying to demonstrate to the dog that you are not a "threat" is not going to make a difference. He already knows you are not a threat. Again, he is being TERRITORIAL, and views you as an intruder in his space. Your dad and his girlfriend have really dropped the ball when it comes to properly training and socializing the dog. A dog must not be allowed to develop the mentality that the house or anything in the house belongs to him. When a dog is allowed to think that way, you end up with a badly behaved dog just like the one you are now living with. The problem is completely correctable, but it must be done by his owners.