Thanks for your additional information; that is really some trauma and difficult situations that your fiance has been through in her childhood.
It sounds like you and your fiance are on the same page regarding any future children having no contact with her mother, and I am unclear if she lives close (in proximity) to you or not? If you all are within the same town/city or state, you may want to bring up a scenario of "what if's" with your fiance, just to make sure you both would be prepared if the (potentially harmful) grandmother re-entered into the picture.
I'm so happy to hear that you are thinking through some of the "worst-case scenarios", and that your fiance is doing well. I wish you both many happy, joyful years together! :-)