Allowing non-housebroken dog to move in

Any dog at any age can learn a new behavior. However, some dogs are more likely to respond to learning than others. In this case, I suspect you will have quite a challenge, as you have three issues working against you. One is the breed- Chihuahuas can be stubborn and not very bright. Two is the accepted behavior that is now unacceptable has been ingrained into the dog its whole life, and you are asking for a change not just to learn something new. And thirdly, you need absolute consistency- everyone must be on board into managing the dog to go outside, and I doubt your mother will cooperate.

Confining the dog to a very small area or crate and taking it out hourly, after eating, after waking from a nap, and any activity helps. Restricting water intake later in the evening helps over the night.