W.C.
I’m so sorry that you are going through this. I went through something similar a couple of months ago (without the man problems as we are both happily married). I know that she is the God-mother to your children, but that is all the more reason to put some distance between the two of you. She is supposed to be an example for them and that can’t happen with her current behavior.
If I were you, I would cancel the trip. You are setting yourself up for a failure here. Tell your children that some things have happened and you will not be able to see this family for now. If they want to know what happened, all you need to tell them is that these are adult issues and they need not worry. The kids can write to one another in the mean time.
She is obviously threatened by you and suffering some self esteem issues, but these are not your issues. She is an adult and needs to be responsible for herself.
If you want to be friends with here right now, do it WITHOUT the children. Take everyone to a sitter and go out together, but the children see things that we don’t think they will and often read much more into it.
Good luck and big hugs.