Agree with the above. iam1butterfly is correct to keep the communication open and friendly. Don't cut her off completely as then she will become angry with you and the relationship will be more difficult with your step daughter. I think you can just be vague when she questions you about personal things regarding your fiance. And slowly extricate yourself from these intrusive conversations. Try to keep the contact friendly but to a minimum. But do it slowly so she does not become upset or mad.
Good luck with your fiance!
Keep an open line of friendly communication as you will need to establish and maintain a civil relationship for the sake of the child (your future step-daughter). However, I would draw the line there... and not divulge private and personal information about you or your fiance as it's really none of her business. To make life simple for all involved, it's best to keep a respectful distance.
I would cut back or eliminate contact with her. That seems like it can do nothing good, in my opinion. You can respect her as a mother and a woman, but you don't have to be friends with her. Hopefully then she won't be such a presence in your life-- there's no need for her to know the details of your relationship. Easier said than done, but I'd give it a try and hope it goes smoothly.