I'm sorry, Stung. I wish I had answers but I'm right there with you... except 20 years later. It is an issue with my kids. Not oldest ds but both of the younger kids. Youngest ds is sarcastic like his father, and makes belittling comments under the guise of 'joking.' I am dealing with it. We are making house rules now that AH is gone and treating each other kindly and respectfully is top of my list! I can't control what is said while at their father's house, but I can model respectful/kind behavior and hope that my actions speak louder than his words. You are definitely right to be concerned about it.
On the counseling thing, I am shocked your counselor allowed your mother to sit in all the sessions? Occasionally, I will attend for part or all of an hour with the kids, or talk to the counselor by myself after their session. But, the majority of the sessions, the kids spend the entire time alone with the counselor. Wow.