Cripes OP, I think the replies are very harsh.
You have a long standing engagement.
She has a sister who can collect her.
I wouldn't have dreamed of imposing on a boyfriend when I had a sister around the corner.
Does anything that comes up in her life trump yours?
If she was very stuck then yes, it would be nice to step in and do this.
Has she scheduled this at her convenience?
Could she have left it until the following week?
I get we are all different, but I certainly would never dream of asking someone to cancel a long standing plan when I had another viable option.
She might be testing you.
Conversely, you could view this as a test for her....and she's failed it.
Being considerate of other people and their plans is important in a relationship.
You think she has pulled a stunt so that you won't see your ex....it sounds like it to me that she has too.