I think you sound like a very kind empathetic woman, but also maybe a little lost.
I was in a EA relationship, I may be projecting so if I am I appologise but it might be possible that what i say may ring true for you.
If she is in love with him, she might not see it even if he had a neon sign above his head, it might have been you would have been the same until you realised what was happening.
If you are still a bit lost or shell shocked I think it might be that this your way of trying to make sense of it all, and that it is likely the feelings you have or feeling sick and unable to sleep stem from the abuse and times you went through, by saving her it almost feels like your trying to right the wrong that happened to you.
I may be well of the mark, but I was so lost after what I went through, despite escaping it. I think I would have felt the same as you.
Even if that doesn't ring true to you, I do think you need to be kind to yourself right now and your children. You can't protect everyone from him, but you can focus on you and your children and try to come to terms with what has happened 
Nobody deserves to go though this, but sometimes we need to be selfish and I think it will be harder for you if she doesn't listen to your warning.