You've got a problem; an XW that doesn't completely act like an XW. You've kept up a good friendship which allowed her the opportunity to never really understand she's divorced. It's odd that she would think your mother would be receptive to her. It is, on occasion, possible for the relationship between the X and the in-laws to continue, but not when the X hurt their son.
Let me ask you something. If you found out that your XW was dating, and serious, how would you take that? Like a friend? Or like a jilted XH? You've got some of the problems I did that are getting a little better...too close to the X and too convinced of a future relationship with her that goes beyond mere friendship.
I don't think you should ditch the relationship you have, but I wonder if you need more space. Space for you to grow without her around, to get comfortable with the concept of a life that doesn't include your X. I think you'll appear more attractive to your XW if you aren't so obviously still available for her.
In the end, it's not the years in your life that count. It's the life in your years. Abraham Lincoln
It is hard to fail, but it is worse never to have tried to succeed. Theodore Roosevelt
