She thinks I am a relative that has come instead? How do I respond to that? She tells me that I was not sitting with her the day before and that i had gone off for some time and one of my relatives has come in and was in the house with her. She thought I was my dad and told me to leave the house and she would call the Police if I didn't and called me a bunch of foul names.
When your wife thinks that you're a relative have you tried to just go with it? Tell her your name and let her think that you're a relative. If she insists that you were not there the day before say something like, "You're right, I wasn't here yesterday, I had some errands to do" instead of disagreeing with her.
Don't try to hammer home who you are because it sounds like it agitates her. Say whatever you have to say to keep her calm in that moment.
My Mom was thinking my Dad was her brother. I told Dad whenever Mom does this, just play along, as it will be comforting to Mom and easier on you. It took awhile for Dad to do this as his first thought was to try to correct Mom.