My stbx does that too, lies about things no one even cares about. I don't understand that behavior, I think it's a symptom of some kind of personality disorder. It seems like it's much more effort then just telling the truth in the first place. I try to ignore it as much as possible. I'm sorry she's setting a bad example for your children. I think the only thing you can do it be the best role model possible for your children. Sadly that's about all you can do because unless she goes to therapy she's not going to change.