Hi Lucky - I can identify with your frustration! I'm a stepmom for 12 yrs now. I met my husband several years post-his-divorce and I almost took a total pass on a great guy since I could see his ex was going to be high, high drama over their 2 kids - indefinitely. Despite the fact she remarried a week after their divorce was final and split mondo assets, I doubt she will ever stop gunning to create negative dramas with him. It as if that's how things are hardwired when you're dealing with a N.
Unfortunately, my impression when I read this thread is that interceeding (advice, therapists) between your SD and her relationship with her NMom seems as if it will meet with a pretty low probablity of success... On the other hand, bolstering your SD's relationship with you and with her Dad is definitely a winning approach. i.e. if she wanted to go to therapy to explore getting closer to her dad, then great.
We're been on the receiving end of constant major and minor parental alienation stuff for years. Badmouthing, fabrications, kids refusing to visit, you name it. A previous poster in this trhead is correct when they said that refuting these allegations (to the children) actually ends up making you guys look that the bad guys since you're pretty much calling the kids mom a liar. Really sad but true.
I sat next to someone a flight and they asked if I was related to ____ (NM-ex), and I said she was my husband's ex, and they gave me the wide-eyed "really???" look with a big wince and I had to laugh. It was weirdly interesting to hear a few more smear campaign anecdotes from my seatmate. It also confirmed that nothing has changed after 12 years.
So, making things safe and welcoming for the kids to enjoy their time with you and their dad without rehashing any NM drama is super hard to do, but it has been the best we can hope for. We've gotten into the habit of changing the subject and consistently encouraging the kids to talk directly to their mom whenever we get traigulated into whatever the latest meltdown or allegation is. We've seen my husband's (and my) relationship with the kids improve as they have grown older and are able to occasionally think more for themselves, but a whole lot of time was lost and a whole lot of tears were shed over the unfairness of it all.
Hope this is helpful, don't want to be preachy, just want to pass along some of what worked for us.