Saying no is very hard, but is often the right thing to do - even though it may not feel like it sometimes!
It sounds like your brother has got so used to leaning on you that he has forgotten that he needs to help himself. Anything you can do to make him see this is a positive, although I would fully expect him to lash out at you in the beginning. Just don't let this break you! Remind yourself that he's not angry at you - he's angry at himself and its just coming out directed at you instead.
Is there anything you can do to help him help himself make his own money? (I don't mean like print it himself - haha).. You know.. any work you can suggest, or a plan to save anything he gets now? Or maybe he needs to focus on not spending what he has? Can you encourage him to find another distraction?