I would NEVER allow outside people into my therapy sessions and would promptly shut down a therapist request for it. Therapy was my sacred space and I wouldn’t want toxic family members to pollute that.

Perhaps as an alternative, could you engage a family therapist for this purpose and leave your regular therapist for your day to day stuff?

Also, I don’t see how arguing with family members about what did or didn’t happen is therapeutic. What is important is how you remember it and how you felt about it at the time.