I know how you feel. Because my M was so needy and enmeshed, I have tried very hard to follow my children's lead when it comes to what sort of relationship they want with me. Then, I miss cues I should have picked up, or I don't listen to my own gut, or I simply can't read their minds and I don't step up when I should. With my SD, there is the added complication of her loyalty issues with her M. When she is on good terms with her M, I am the evil SM, but when things are bad with her diagnosable M, I am golden. Right now, she isn't even talking to her M, and is trying to pull me back in, but I don't trust it. In a year, we have gone from her not being able to tolerate talking to me on the phone, to her initiating contact. I am cautious, and will probably remain cautious with her for the rest of my life.
Stepping up the contact is a good plan, but be prepared for fall out. People who are as enmeshed as your DIL is don't even realize how unhealthy their familial relationships are until there is a crisis, like say your DS finally getting tired of it and insisting on change. If they do finally see it and they try to overcome their lifetime of training, their family will put them through pain and grief. I should know, mine sure did. You know what you went through because of your DS's marriage, well her family really hasn't had to change that much. When they do have to change, it will be WWIII and you will be a safe harbor for both of them.