You have acknowledge that Si is a parent to the kid, that she considers them her own, and that the one who is old enough to also considers her his parents.
Now that she's pretty much banned from your house, without visitation rights with her children, have you arranged for the children to spend time at her place on a regular basis? It seems only fair that she doesn't get separated from her children just because one of the other parents hates her now. Put yourself in her place, I'm sure either of you would want to still be in touch with your children if you were suddenly kicked out of the relationship, and it should be the same with her.
My ex had similar communication issues, too, by the way. He would tell me what he thought I wanted to hear, and resent me for it. Most of the time I adapted to what he told me, which wasn't always practical, when the truth would have worked better (and not just because it was the truth, either). So in the end it was the worst case scenario for both of us.
Annoyingly, because he also did this thing of pretending everything was fine when it wasn't (and expecting me to guess and act accordingly) he would constantly bother me when I wanted to be alone.
I would tell him "I want/need to be alone", he would assume I actually needed him around (because that's what he said when he needed me to stay. It boggles the mind).
And so when I needed solitude to process things or deal with stress, he would instead bother me until I got so upset of repeating I needed to be alone that I would kick him out of the room or go out of the house.
At which point he always seemed like a lost puppy, or like I had just betrayed him. He had done "everything right" and yet I was still upset! It was so unfair!
In other words, communication style is very important. Months after we broke up it turned out he wasn't poly. He told me that I should have realised he was only faking happiness. He resented me for it. Yes, you read it right, he resented me for trusting him, for not constantly assuming he was lying to me.
This is a pretty ridiculous situation. Please communicate what you actually feel and think, and treat what people say at face value. Because even if you know they're lying, then you have no way to guess what the truth is, among all the different possibilities. The only way to know is to be told.