Well hopefully the thing with the car will get straightened out on Monday. As for Matt and Si, I don't know what to suggest when your daughter asks why Si's not around anymore. Either, "Ask Daddy, he will explain it," or, "Dad and Si are mad at each other and Daddy wants Si to go away." Ulf, those both sound pretty awful, but those are about your only options (shy of fibbing to your daughter which would, like, be worse).
A good solution would be if Matt and Si would both apologize to each other for the sake of the child. Obviously, though, that's not something you can control, and a good reason why you shouldn't be put in the middle of this.
It's true, things could be worse. Maybe once you guys move, this problem will kind of go away. Just do the best you can with your actions, which you can control.
Love means never having to say, "Put down that meat cleaver!"