What I see here is two people divorcing who are bitter and hostile and each trying to recruit their kids into taking sides. It's an old and familiar story that brings credit to no one.
Divine 1966 gave exellent advice about limiting ex-wife's access to your home and having her contact with her daughter be court-directed. Or do you prefer that your ex and daughter meet and converse under your roof, where you can listen in on their interaction?
Give your daughter a little credit for not being a total fool. Kids are pretty good at figuring out who loves them. It's not your place to tell your daughter that her mother doesn't love her and is only throwing breadcrumbs her way. That would constitute alienation. Aside from the implications in court that you do fear, it is just wrong. DD didn't get to choose who she would have for a mother. You made that choice for her.
If your ex is the monster you portray her as - which I'm not disputing - one has to question your judgement in marrying such a person. Maybe your judgement is less than perfect, all-knowing and all-wise. Maybe it's time for a little humility, instead of this high-stakes competition over who the kids will side with . . . and who the therapist will side with. Therapists are not supposed to make decisions for their clients.
The therapist telling DD to have no contact with her mother sounds out of line to me. If you are paying for all this therapy, perhaps the therapist is courting favor with you. Therapists have families to support and bills to pay, just like the rest of us. This whole family therapy enterprise sounds like a non-stop quest to demonize your ex. and satisfy an insatiable hunger for validation. Apparently, this therapist has been delivering the goods.
Your daughter is under intense pressure to conform to other's expectations of who she should love and whose "side" she should be on. That's unhealthy for her. It's apt to mold her in ways that you won't like. It could lead to her feeling she has to put on a false front and change it, depending on who she's with. This is how you teach a child to be manipulative. Sounds like her brother has already mastered that.