I just make sure my sons are safe; beyond that, there isn't anything I can do.
Doodler,
Does she get them, too? How can you be sure they aren't being introduced to "bad" things while with her? Forgive me, but I've forgotten your kids' ages. Mine are 4 and 6. I'm not so sure she isn't doing some sort of parental alienation. I'm hearing talk from the only family member of hers that is left on my side that her and her sister are up to something as far as the kids are concerned. That scares me.
There are moments in this life when you are so confident in the rightness of your actions, that not even for a second do you consider the option that you might be wrong.