My husband and I have been married 21 yrs...1 yr ago we opened our relationship to T..it was my idea...T has been my best friend for 10 yrs..we all live one another very much..have worked thru jealousy issues etc..the only obsticle we face is a bitter resistance from T adult child..what do we do?
I think you could become ok with it. That the adult child can feel however they want to feel. I assume they live in their own home and are doing their own adult life things.
What Dad (?) T does in his own home own adult life is his deal. They don't have to like it.
T could state his expectations. That he will continue to invite the adult child to family events T hosts, and hope that the adult child can behave politely if they choose to attend since his other partners will be there. If they cannot, T can expect them to simply decline and RSVP "No thank you."
Let the adult child be responsible for their own choices and their own behavior.
I am not sure if your partner T is male or female. Or if their adult child is male or female. It doesn't matter, really, I guess. Except that if T is female and her grown child is male, he might feel protective toward her, and feel that she isn't being served well by not having her "own man." Maybe he thinks she is just a side piece.
Be that as it may, most of us struggle when we "come out" as poly to our families. Some family members can see us happy and just accept it. Some judge quite harshly and never get over it. But quite often, many who were disapproving at first, come around in a year or two. Even if they don't fully understand it, their love for us softens their position over time.
It hurts to "lose" a child. It can happen over other differences of opinion, life choices, over personal definitions of "morality." I am sorry it's been hard for T and for you.
It saddens me that T's adult child won't accept T's life choices. I'm not sure that there's anything you can do. Let T's adult child live their own life, and you (and T) live yours. Include T's adult child only if they will act decent toward the three of you.