Dear Candy,
Other people on here will have better advice than me. I have no siblings, nobody else interfering in my decisions, and at times that is hard for me having to do it all myself, but certainly better than having other people with conflicting, greedy, and uncaring motives. I don't understand their mentality. Do they not have consciences?
Can you get somewhere for your mum to go for a spell of respite care? You choose it. While she is there, contact the Court of Protection to see exactly what the score is. I can't believe they haven't experienced this kind of thing before, where one sibling uses a parent's money to suit himself. The will surely want evidence from him of his intention in spending the money.
You can't do it all on your own, not with this aggro from him.
Having no siblings I find it difficult to advise. Just to give a parallel story, my daughter's boyfriend's mother has had motor neurone disease for 15 years. She is now virtually paralysed (including her swallowing capability), her husband cares for her 100% at home. Living at home are two sons and a daughter and they do - nothing. My daughter's boyfriend has only just discovered the lack of help they give - he has been cooking his dad meals to eat during the week to make sure he eats properly. He went on Christmas Day and did the whole works (never done it before) and his siblings did nothing to help him. He asked if they could please cook one family meal a week each to ensure that dad gets a decent meal. Answer "No, why should we bother?".
Someone with siblings will have to answer this better than I can, but my heart goes out to you. Let us know how you go on, you sound at the end of your tether.
Love
Margaret