You know, I know someone who is in a situation almost identical to this. But there are some extremes in play. The user (not sure what his drug of choice is) is 45 and refuses to out of his mother's tiny condo. She's in her 70's and works fulltime to pay for groceries and support everyone. Her poor husband is also in his 70's and has emphysema. Everyone in their family is so absolutely disgusted with this guy. He wanted a carpet cleaning business, so he talked her into getting a loan for $3k to pay for a truck. Then he didn't like it and she took a big hit on the truck. Now he wants to buy a hot dog stand and sell hot dogs. The guy has lived with her the most of his life. He is just the biggest loser.
Meanwhile, her grandson (son of her other son) has just signed a professional sport's team contract for over $20 million. Her other son is worth millions in her own right, as are a few of her other offspring. The really odd thing is, if she'd kick her loser son out of the house, they have all offered to buy her a nice place and support her for the rest of her retired life. She'd have it made!!! But she feels she'd be turning her back on her son, who just drinks beer and watches tv all day.
Nobody can understand why she has continued to lose her sanity over this guy, cripple him with codependence and succumb her husband to this guy's insanity. But then again, nobody's trying too hard to figure it out. It's her deal. So, Grandma works fulltime and can't visit anyone much because she only has 2 weeks a year vacation, and can barely afford the basics.
So I totally see where you are coming from. The favors never cease to end. And in the case I cited, I can see where now the guy's mother really does worry that he'd be homeless because he just simply doesn't know how to be independent. He's only really lived in rehab if he wasn't at her house.
But I think you are really better off if the more people who agree not to continue enabling your mother as she enables your brother, the better. Because if the situation remains comfortable enough for them, it will continue. I know it has to be hard watching your mother care for him like she does.