Hello, it seems you may be offline at this time. When it comes to a sudden behavior change such as this, you want to have a vet check if you have not done this I would at least drop off a first of the morning sample to your vet to check for a UTI or Bladder stones. Depending on the breed , some dogs are more prone to stones than others.
From a behavior standpoint, sudden changes can happen when there have been changes such as a visit from a person or pet, a change in the shift of power between the pack of dogs, or separation anxiety.
Dogs that have been potty trained to go outside usually do not want to mess in the home as it is considered their den. So something is going on.
Try thinking back to when it first started, what was going on between the dogs? Were there any visitors,. etc.?
At this point if it is strictly behavior then she cannot have free roam of the house. She should be crated, tethered or gated in an area near her food and water . To allow her to have free roam at this point is setting her up to fail.
You will need to try redoing potty training but make sure the reward for going outside is one she will remember and only get when she potties outside. hot dog or chicken slivers usually work well for this. I would also train a potty command so she goes when you ask her.
But do take a sample of urine in for testing and take notice if she is thirsty more than usually as diabetes is another problem in dogs.