If you're so far away then I think it is difficult for you to know enough about your dm's care.
Both my parents ended up being the closest child to their parents when they needed care. In both cases siblings thought they weren't doing the right care until they came and saw for some time what was actually going on. In both cases caring put huge pressure on them, which the siblings didn't ever fully realise.
In dm's case if her siblings hadn't trusted her, then they probably could have accused her of spending their money. In her case what used to happen was dm would go to the supermarket, think "oh they need some more X Y Z" and take it round to them later. So she'd then arrive, put the shopping away, change the sheets, do a couple of meals for the freezer, empty the dishwasher.... and dgd would then say to her "I was thinking, could you get me another pen next time you go to Smiths. They're so good, get yourself one too". and give hert £10 to get two pens.
So dm would have spent £20 on shopping, 3 hours on caring and get a £5 pen out of it. Good thing she did it for love wasn't it?
But if the siblings had seen she'd taken £10 and bought a pen for dgd and a pen for herself, they could easily have thought she'd told him pens had gone up and pinched the money/used ti on herself.