It may violate their professional ethics even if, in theory, waivers could be signed allowing the meeting. I know my T has refused to allow some people from my life to come to session but is very welcoming of others. He does state it as a professional ethics boundary... it is complicated but I can see what he is getting at.
One example was my son as my T 1. Did not feel competent to deal with my sons mental health issues 2. Felt that becoming involved in my parenting might threaten our other therapeutic work/relationship 3. That my son might become destructive and again, the dynamic may impact my work with T. T did however let my husband come to a session.