Mentally ill brother has a drinking problem?

My older brother has been drinking since he was 21. At that time, our dad was alive and used to go with him to the bar for a drink. That's when everyone believes it started.

I don't discredit that, however our dad has been dead for 8 years. His drinking has only got worse. My brother is a paranoid schizophrenic, and his nurse is always telling him not to drink so much, as it's clashing with his medication. There have been times where his medication didn't work, and they had to change it. I know a person can become immune over time, but is drinking speeding up the process?

He constantly lies, asks for money (he becomes moody if he doesn't get it, repeatedly asks, etc). He has said that drinking makes him feel good, and that he doesn't look good if he doesn't have a drink. He has said he wants a drink to settle his nerves. He has become tolerant to alcohol, as he no longer gets hangovers.

My mum and brother are now becoming secretive, she being secretive about giving him money, and him lying that she never gave him any. They do this because I'm the only one who believes that she shouldn't encourage this behaviour. He asked for money yesterday, bought six cans of beer at 10:30 AM. They were gone by lunch.

I've warned him today that I will call his nurse tomorrow, and tell him about the money, and also that I suspect a drinking problem. My brother turned around and said: "Don't tell him. Tell her not to mum! Don't tell him, I mean that."

So, I don't know what else to do. Risk calling his nurse, and having my brother get angry, or let him carry on this path.