Just an update (and I need to get this off my chest!) - Apparently "no" means "Yes, absolutely" to some people.
Despite having to say no TWICE, I just found out today that the SIL was still planning on coming over to my place this Saturday. She seemed surprised that I said I wasn't going to be home. I told her she confused me with what plans she was talking about for Saturday because I had already said no. She said fine, she will cancel it then. Of course, she's said this before though!
Seriously needing to make sure I am out somewhere Saturday night now hahaha! What a nightmare this is turning out to be!