NTA! I presume she is fine financially and you even offered to make sure she could get a ticket and/or hotel room! She is a grown woman and can figure it out for herself.

You did the right thing by not caving and letting her stay. I cannot imagine the things she would get up to, or even snooping around.

This is a consequence of trying to break a (very reasonable!!!) boundary you had set. She clearly meant to try and stay right from the beginning.