You don't seem to have asked for help with anything specific, apart from saying that your Brother sometimes off loads how difficult it is being your Mothers main carer for the great majority of the time.
If I was you I would sympathize with him and tell him that you appreciate how difficult it is, as you yourself help for a few days every month.
What I would say is, make some enquiries with the Carers about her weight loss and lack of food in the fridge and see what they have to say. As others have stated, if her meals are delivered then she won't need food in the fridge.
The other point you made about your Brother being an alcoholic, he may well be, but he could be a functioning alcoholic, there are many who manage to hold down full time jobs who are most probably alcoholics.
If what he is contributing is keeping your Mum in her home and reasonably contented, then I wouldn't be causing too many waves.
From what I have seen of people with dementia in care homes, the majority of them are pretty unhappy, wandering around asking to be allowed to go home. They have their day to day physical needs met but I feel most of them would have managed better if they could have stayed in their own homes, in familiar surroundings with all their memories around them, giving them some comfort.
What I'm trying to say is, for your Mothers sake, this is probably the best option, albeit not perfect.