So let's say Matt did a total 180, and he and Si made peace. Would she then go to your new home? Truly wanting to understand the dynamic, I understood that all of you agreed to move. Then Si came to you and you made new plans to stay in UK with her, without discussion with Matt.
I am not sure if I understand that correctly.
Also when Si, decided to ignore the children after the major fight wasn't that hurtful. It seems like this is not healthly for a child either.
Perhaps explaining like any parent, would that Si was part of your life but that she is moving on to other things and while she will always be in her heart, she has made the decision not to move. Honestly, I would be more concerned if the Nanny hadn't gone. Matt, you and the Nanny appear to be the solid foundation in your children's life. Si seems to be there for the good but not the bad.
I do have something to compare this to. My sister was very involved with my child on a daily basis and was very intergal to her. My sister did something that betrayed myself, my husband and my child. She tried to break our family unit out of greed to have a child, herself. I had to remove her from our life and it was difficult on my child. However, I do believe this made the three of us closer as a family and brought a view that people aren't perfect even if we love them to my child.
Truthfully, I have few memories of that time period in my life. As fast as life changes my daughter has few memories and that includes some traumatic stuff from that age.