It does sound as though the therapist isn't hugely helpful. Surely the way forward would be to reconcile the two parties, not cut each other out of their lives.
I can well imagine that your sister was hurt by her daughter's accusations, but I think your sister should still acknowledge her child's distress about the situation even though there was no other choice. It's understandable that you sis wants to defend herself but I think if she can say to her daughter 'I hear you what you're saying and I understand why you're upset,' it might go some way to healing.
The next thread for friendship, advice and support if estrangement has affected your life