K.N.
You ought to tell her that her behavior is hurting your esteem and admiration for her. That she is jeopardizing your ability to trust her, that you see her actions as harmful to other members of the family. If she continues, after you have told her this, then distance yourself from her. She needs to tangibly know/feel that you do not support or condone her behavior.
That they get a divorce is separate from you. However, even when they are divorced, they still are members of a family... She either needs to realign her values and priorities so that they are not harmful to your family... or you and other members of your family will need to distance yourself from someone who is creating a toxic environment and threatening its stability.