Why did you stay with Dad?
For all the problems my mother and father had in their marriage, their resolute commitment to their marriage vows, in spite of their challenges, remains one of the strongest messages about marriage I received from both of them. In moments of frustration, I would ask my mother, "Why do you stay with Dad if you're so unhappy?" Her response was always the same, "I love him and I married him for life, no matter how he acts. He doesn't mean it. He doesn't know any better."
What's interesting to me about the question in this case is not just my or your mother's answer, but also your motives for asking it. If you're like me, you already know the answer. The question is a mask for anger. For me it was anger at my mother for allowing herself to be treated so poorly, at my father for treating her so poorly and at myself for not being able to fix the situation.
The question can also be an indirect way of stating how you really feel. If this is the case for you, I suggest you first ask yourself the question, "Why do I think Mom is staying with Dad?" Once you know the answer to that question (because she's afraid to be on her own, because she has no income, because she takes her marriage vows literally, because there are intimate details she's kept private or some other private matter), address the issue with her. Make sure you have already established a comfortable rapport with her during a calm moment perhaps driving in the car or on vacation or during a quiet afternoon walk