http://www.soberrecovery.com/forums/...oundaries.html
The above link is a link to an a post about boundaries. There's also a link in my signature line. I'd recommend you start reading all you can about codependancy (Codependant no more by Melody Beattie is a good one) and read and set boundaries.
Seems to me that you've been trying to set boundaries (or bargain) but you have no consequence for the broken boundary.
It's like this (as harsh as it sounds).
You have your life. Your mother has hers. And your brother has his. Each of you are responsible for your own actions, decisions, etc.
So if you are truly ready to stop the insanity - you can! Anytime you are ready.
And you have to learn to realize that you are not responsible for your Mom's feelings (or guilt trips). If she chooses your brother, than that is her choice!!
Has your Mom ever attended any Alanon meetings? Have you? Maybe that could be a good start as well.