A big issue for me would be the age of the girls when this happened. If they were in their late teens or later, I would say they share some culpability. If the girls were younger, then they could be guilty of lying and making stuff up, but more likely the therapist misinterpreted stuff they said, and it got built up and up until eventually there is a semi-memory of the incident.
It's a different situation, but when I was 6 years old I got lost in Acapulco Mexico on a vacation. My older siblings were told that if they went to the beach they had to bring me, they didn't want to, so they ditched me. I wandered alone at 6 years old for the next 5 hours before I was found. I had a specific memory from this of talking to a guy selling jewelry out of a briefcase when I was found, my parents were so happy they bought me a turquoise ring that I liked. This story was told so many times over the years, and each time, in my mind, little bits of information built a false memory of the incident. I know this because I was going through some old boxes at my parents house and came upon the ring my parents bought me and a picture of the guy we bought the ring from. First off, the ring was all silver, no turquoise on it. Second, the picture showed a man very very different than my memory (which was a very stereotypical Mexican beach bum) and that merchant didn't have a briefcase, but actual display cases laid out neatly on blanket on the beach. My point is, memories are not perfect but if repeated enough times, become totally real. If someone wanted to bet me everything I owned that my ring had turquoise on it, I would have jumped at that bet. Each time we retell a story, we solidify the facts of the story into becoming a strong memory. And if those facts are incorrect, then our memory can be incorrect.
Did that make sense, or was I just rambling.