Hi Monty, I think this is such a difficult stage. When my mum used to say it wasn't her house, at first I would point at things & remind her. Over time, though, I realised that just made her more agitated.
I'd go along with whatever explanation she came up with. If she said she'd only come for a holiday, I'd say as she'd paid, she may as well get her monies worth.
Sometimes it worked to say her house was having work done.
When your mum says she doesn't live there, she knows it to be true, so you won't be able to convince her otherwise.
Go with the flow, really, it's the kindest thing to do.
Lin x