Sorry that you are going through this. You didn't mention how old your brother is but I assume he is of legal age?
You can only offer advice to your mother but you can't make her take it. I am the mother of a nearly 27 yr old daughter and believe me, it took a lot for me to come out of "denial" over her drug use. I made all sorts of excuses for her, hoping that whatever she was doing, she would just grow out of it. It was when my younger, sober daughter turned to me one day and said that I was so tired and caught up in her older sister's life that there wasn't much left for her! that I realized I was hurting the very one who really needed my help.
Addicts do not seek or ask for help or even get help because of someone else, no matter how bad things get. They will only do it because they want the help and are willing to fight for it. Your brother does not sound like he is ready by any means. And yes, your mother allowing him a safe place to crash will only delay the inevitable. It won't help him to get help at all. All it will do is continue to make her life miserable and keep her raising the "expectations" of her son's behavior. She really needs to accept that he is not there even when he is there. She needs to focus on her and her life and since he is not providing the financial help she counted on, she cannot continue to expect that. And an addict will NOT get clean or go into treatment as part of a bargaining chip. It might work for the time he is in treatment, if he were to go, but it would not last.
I'm glad that you are trying to stay away from the roller-coaster. You are not the cause of what is happening and all your involvement will mean is that you, too, will become a victim of the addiction.
You cannot control the addictive behavior of an addict; you can only control how much you allow it to control YOU.
Maybe you can find out if there is an alanon or naranon meeting in your area. Even if you can't get your mother to go to one, you can bring home some literature that she could read. It's hard to get someone to see things through your perspective no matter how much they are allowing themselves to get hurt. But remember, you did not cause this and you cannot control this and you cannot cure cure this! (The three C's)
Keep coming here; you are not alone.
Hugs,
Marteen