This sounds like a situation in which the probate court has to get involved because they are the ones with the authority to appoint a conservator for the person and property of your brother.

They have investigators who will interview you and everyone else involved including the medical staff and your brother's estranged wife.

Based on what they find, they will make an order appointing you (or a professional conservator) as the one to look after your brother's care.

I am very sorry to hear about your situation. This is the time to request all your friends and family to come to your aid. Talk to the estranged wife, but if she's callous then ignore her. Do for your brother what you think he would want people to do for him.

In our city, we have what's called an adult protective agency, and other such organizations that are familiar with situations like what you described. You could see what help is available locally where you are situated.

I'll hope and pray for a good recovery or care for you and your brother.