I'm so sorry Polly, what a terrible shock and violation of trust. Your ex-gf has major problems and should seek help.
But you couldn't have known. You are not to blame. Many women who've lived perfectly "normal" lives -- hetero, mono -- have had to deal with the awful reality of their partner trying something with their kids. Sometimes people are just damaged. It doesn't mean that what you and she had wasn't real, just that there was a damaged side to her you didn't know about. You did the right thing by cutting her loose right away. As sick as it makes me to say, not every woman in your position does the same...
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.