First, very sorry to hear about your mother. That's unbelievably hard. But your brother is holding your whole family hostage. You are in a tough situation but somebody has to take a stand here. You say he IS taking medication? It doesn't sound like it is working. Does he see a psychiatrist? In therapy? He needs to be. Let him fight all he wants, don't give him money. You are not obligated. If he cannot earn money due to mental health issues, he can apply for disability. But he should not expect to be taken care of for the rest of his life. This snot your father's obligation and especially not yours. And his living this way with family that enables him to be dysfunctional just keeps it going longer.
I would set a time frame for him to be employed and for his moving out. And be ready to move his things into a storage unit where paid for two months and then you aren't paying. They'll lock it and he'll have to pay to get his stuff. Sounds harsh but this can't go on.
If someone isn't going to TRY, there isn't much you can do to help them. But I know this is really hard and may seem impossible to do. Just know that you will ultimately be helping him as he is forced to overhaul his life. good luck