My personal and immediate reaction is that this is why it is dangerous to let people know you're in therapy. If it were me, I would feel extremely uncomfortable (and I'd definitely not bring it up with the family member - I'd make certain that I never discussed anything remotely personal with a person who would gossip about me.) I see no caring at all in that kind of interference, and to me, just because somebody happens to be related to you it doesn't mean that your private business is any of their business.

I think bringing it up with your therapist sounds like a good plan. A T should be on the client's side, and not collude with other people in order to allow the client's privacy to be invaded.