My parents are very supportive and came along to a couple of sessions with my old pdoc. All it achieved was them seeing what a jerk he was, which was helpful to me as needed that confirmation. My parents have also seen my T and it really helped them as he is a brilliant T and was able to answer their questions.

However, in the past my T met with my husband and his parents. I was in conflict with his parents at the time. It was a train-wreck. Again it helped as my T could confirm to me that my parents-in-law were crazy (in a bad way) and that I should have as little to do with them as possible.

In your case I agree that it is best to find out the full purpose of this meeting and if you feel uncomfortable say no. You don't need to put yourself at risk.