Surely she runs out of people she knows who are sick and dying? But yes, I'd stop her in her tracks before she gets too far and tell her if she continues you'll have to leave/end the conversation. Some people don't "get it" and have to be "trained" before they know you're serious. You don't have to put up with any conversation from anyone you don't want to, doesn't matter that it's your mom. Good for you taking care of yourself!