I would sit down with your future DIL and have a chat. My MIL and I have found that really opening up to one another has helped us heaps. I had a XMIL who I had a lot of issues with and I brought those into the relationship with my new MIL so it was great to b able to discuss them with her and work through it. MY mIL had a really really awful MIL so that has made her determined not to be like her LOL
If you feel uncomfortable talking to her I think writing is a great way of addressing the issue. Its a very sensitive topic and I know I was very worried when DH and i were first together that his mum would still favour his X (they had no kids at least) The best thing for me to get over it has been time but I know I was very paranoid and jealous to start with. You need to make your new DIL aware that you are NOT friends with the X and tolerate her for the sake of the child.
Good luck! You sound like a lovely MIL!!