If it wasn't odd, if it was natural and she was like an aunt to this girl, she wouldn't be on here asking how to say "no" to a child. They don't have the kind of relationship that would warrant an overnight if she can't set boundaries with the girl. They clearly aren't that close.
I do have kids, and over the years have had many nieces and nephews spend the night. I can say no to them and set boundaries for them when they are under my care. This is not the same situation.