SOunds like you could be/ are dating a formerly (?) abused / still abused (?) person.
Could familiarize yourself with that website, especially the tactics and the "stages" vicitms go through.
She could be struggling with coparenting with her abuser, being in a woman-woman thing (given how the ex puts her down for it and tells he she is crazy) AND being in a polyship in general and worried about your spouse.
Any one of those single things is a lot on the stress scale. Combined? It could feel like geometric stress to her, not just additive.
She could be concerned about him playing the kids against her and threatening custody issues.
Could tread carefully, and could remember to be a true friend before anything else. (girlfriend, lover, etc) Could ask her given her reticence/worries, if it is easier on her to be only friends for now at this time and not date/lovers until a better time. What are HER needs? Wants? Limits?
Go platinum rule here -- ASK her. So you can treat her how SHE wants to be treated.