Getting that diagnosis is so important. It does open doors to people helping you. Our Admiral's Nurse even helped us get a blue badge for my DM (and they are like gold dust). Her reasoning was (and she was quite correct as it turned out), if/when walking became an issue and the car was right on the other side of the car park do you really want the choice of dragging the equivalent of a grown up toddler across a busy piece of ground or leaving a person who may wander off on their own whilst you get the car? Neither one works and that blue badge meant we could, whenever possible, park close to our destination. Which meant less tantrums and less worry. Our Admiral's Nurse explained that, for example, even going to the supermarket would become extremely tiring. Imagine not remembering which aisle the apples were in. Now imagine not remembering what an apple looked like...that's the progression sometimes and, that lack of remembering can come on all of a sudden then snap back. Some people even have the hallucination of doors disappearing so they can't find their way out of a room for a few minutes. All of which adds to the tiredness as your mind battles with itself trying to get that bit of memory back.
POA is really important if you can get it. It meant that I could make decisions on mum's behalf, have banks talk to me, hospitals talk to me, all without mum's specific instructions. As she is also deaf that made quite a difference!
Make sure you and your DSis come to arrangements regarding breaks. Looking after someone full time is hard and often soul destroying - even having an afternoon to yourself sometimes is a big boost.
And if you have day centre get her on the waiting list. They take the person for the day (or 9am to 3pm) and have things like bingo, craft, visits, hairdressers, chiropodists in many cases. Mum was always more 'sparky' when she came back from her weekly visit because of the stimulation.
And if she doesn't get it already, apply for Attendance Allowance. Most people I know who have dementia get it. it's not means tested and your DM can use it any thing she wants - domestic help, even paying for a mobile hairdresser to help her if she can't get out.
Your DSis may be entitled to a carer's allowance too. Best to check.
And finally, start getting everything financial set up on standing orders or direct debits. My mum's signature soon became illegible so cheques couldn't be written and that can be extremely awkward. Get as much as you can on direct debit as standing orders have to be signed as things change - direct debits don't.
Oh and a really daft thing. We got a stamp made of mum's signature for Christmas and birthday cards. I realised she was getting upset not being able to sign cards and she used to have great fun stamping her Christmas cards whilst we shared a mince pie and a cuppa!
Good luck. It is hard and everyone is different. You just sometimes have to think outside the box as they say.