Are you sure you aren't just making some assumptions? How do you know why your mother is or isn't telling you things? Did she sit down with you and say, "Your father is getting worse, but I'm not going to tell you about it because it might jeopardize your sobriety?"
Have a talk with her. See what you can do to help. Seems like her hands are pretty full at the moment. Being of service to others can be a great benefit to sobriety. Open up the communication and see what happens.