You are now considered an adult. You need to try to move on with your life and learn form your parents mistakes. It is not up to you to change your mother, and if you dad has not been able to control her all these years then you have even less chance of doing so, and it will do nothing but cause you misery and forever cause a rift between you and your mom. She is what she is and unless and until they both come to terms with what she and your father for that matter have become and stooped to in their marriage, there is nothing you can do to remedy the situation. What drives them to be so dysfunctional has nothing to do with you as their children. I am sure that they love you and are in truth mortified as the life that they have led, but they will not answer to you for it.
Your best bet is to move on with your life. If you feel you must, then let them both know how you feel about what they are doing, and that you are moving on with your life in a healthy way. That is the best you can do, and I doubt without something drastic happening between the two of them, they will continue.
It is hard to break this infidelity cycle even when one of the spouses finds out and fights for the marriage, but when both partners are ambivilant about it, there is most certainly no hope for a good outcome.
Move on with you life.
Sunny