I wonder whether Molly just doesn't see your parents' house as part of her generalized "den," where it's not acceptable to soil. The long periods outside during the week's travel break may also have confused her. Do you have something you say to encourage her to do her business? We always teach our dogs what "Go piddle!" means by praising them with "Good piddle!" when they go, or "Good hurry up!" when they have a bowel movement. I have to say that that didn't help when we left our dog at my son's because we were going to France for two weeks. She was so traumatized that she wouldn't go no matter what was said to her. But I think you're on the right track to crate her and then let her out, use some trigger word or phrase for her to associate with relieving herself, and gradually retrain her. How frustrating this must be for you!
Leslie
Leslie