in your aunt’s head, they don’t have to leave until tomorrow, so I suspect she’ll withhold the info until then.
But why? She's been asked for the details many times - for somebody who has paid as much as she has to be there - so why can't she respect that and give the info?
As for places that don't release the exact location until the day before, I'd assume this must mean that it's situated in the dodgy part of town right near where the drug dealers hang out and wouldn't book it. If you wouldn't pay a fiver for a 'mystery sandwich' from a lunch shop without being able to see what you're getting, why on earth would you pay hundreds or even thousands for a mystery holiday cottage?
I'm sure they wouldn't be at all happy if the customer insisted on only releasing their payment for renting the place the day before.
Back to the aunt... the problem with knowing that you have it all covered and that you will make certain to pass on the necessary details on a (to you) 'just in time' basis is that other people don't live in your head and don't know if you've forgotten, are being deliberately difficult or if there's some other problem. It's a very basic common courtesy to keep people informed and not to gatekeep necessary information until you deem it appropriate to tell. People look forward to holidays in different ways, and if somebody likes to know and plan ahead for all the little details of their holiday - even if you personally don't really care you're an absolute arse to deliberately prevent them from being able to do so.
My only other wonder might be that she's invited somebody else along and now realises that there isn't now enough room there for everybody. If that were the case, she might be hoping to spring it on your Mum at the last minute that 'there's a problem with the holiday - she'll chase them for a refund' and for her not to go after all. Either that or, rather than her wanting to grab the best bedroom for herself, she could be planning on your Mum getting there last of all, before she can grab any bedroom, as - unbeknownst to her - there is no bedroom for her and she's now on the sofa-bed in the living room.
Forgive me if I've misconstrued this, but I get the impression that - as well as not being tech-savvy - she's quite meek and trusting and liable to be taken advantage of if somebody chose to do so?