You can enable your brother and enable your mother at the same time.
The way I see it, if she chooses to enable him, its her business. If you choose to enable her enabling by falling for the line that you would be doing it for her not him, well that is where you can decide to draw the line on enabling if you want to. Will it make your mother mad? Probably. So? Would you rather keep doing things that you don't want to do just to keep the peace?