That sounds like a total nightmare. My parents both died in 2016, they were in their 80s and both had several health issues. Dad had advanced dementia by the last year of his life, and was in a care home, but prior to then he was living at home with Mum, and there were several hospital admissions where I had to accompany him and explain all his medical history, as he wasn't able to do that himself and the medics were always too busy to read his notes. I felt like I was a walking, talking version of his medical notes, and knew that if I didn't perform that function something important might be overlooked. When your elderly parent has dementia they are so vulnerable that you really become their advocate, and are constantly sorting out issues as they arise. On top of that the personal care side is difficult, for many reasons.
I can't imagine how much worse all that is, when you are prevented from doing that, due to a pandemic. It sounds like your mother has been discharged without any proper care plan in place. Perhaps she masked her symptoms and told them she was coping just fine - that's very common in these situations, where pride comes into play. If your brother can't/won't step up, maybe you and your DC will have to work out some kind of rota to cover the essential care, until you can either source agency workers to help support her at home, or move her into a care home. From what you've said I think this may be a case of fire fighting until the next crisis occurs, then you can call an ambulance and push for re-admission, stating that she has had a failed discharge and must not be sent home again without an appropriate care package in place because it is not safe and you cannot provide the level of care needed. You might need to complain loudly to the hospital management/PALs liaison officer to get yourself heard.
Meantime, buy plenty of incontinence pads/pants, and plenty of disposable gloves, to deal with the toilet issues. Maybe put some old towels under her bed sheet to soak up the worst of the inevitable toilet accidents. I'd recommemd that you visit the forum on the Carers UK website, where lots of people who've been in similar situations will offer support and advice.
You have my utmost sympathy, this is a really hard situation to be in, and you are probably still grieving the loss of your father. Life is so tough at times. Take care of yourself too.