You have to be suspicious of someone who only sees the person with dementia when there is some financial reward for doing so. From what you've said, it sounds like this sibling is deliberating hiding funds/assets to avoid paying out in a divorce settlement. Which is illegal of course... If they've 'hidden' their assets in your mum's overseas account and she dies, legally those monies would form part of her estate unless your sibling's name is also on the account. If that's the case (i.e. an account in joint names) when she dies, all the money would legally belong to the sibling. So I think you need to look at that and try to sort the situation out...
Good luck - you're going to need it.
Good luck - you're going to need it.