Dear mudpup,
I do not know about the dynamics of co-dependency, but could it be that in some bizarre way, your mom is "getting something" out of this relationship with your brother?
I know very little about psychology, so forgive my ignorance---but in situations like this I wonder if there might not be some symbiosis present. I can only offer the perspective based on my own experience: My youngest brother decided to drop out of college and move back in with my parents full time, and when my Nmother told me about this(this was years ago when we were still talking), she said to me that it would "be a comfort to have [my brother] around all the time." My mother does not drive, she has no friends nor interests of her own(as she claims that she sacrificed all of those things on the mighty altar of parenthood---what a load---but I digress), and now that the nest was empty I think she found herself wondering what to do with herself now that there were no young children who "needed" her.
My juvenile brother's full-time presence in my Nmother's home allowed her to go back to parenting on a level that she felt "comfortable" with(and Lord knows, with an Nmother, HER comfort level takes top priority over everything and everyone else), whereas my other brother and I were full-fledged adults who had married and made new families with our spouses(or, to quote my Nmother and Nsisters, we "brought OUTSIDERS" into our family of origin). I think that my mother cannot deal with the fact that she no longer has control over our lives, so she makes excuses for our youngest brother's irresponsibility and immaturity because it suits her own twisted needs(his neediness strokes her ego).
As far as your mom's willingness to put up with your brother's abuse, the only thing I can offer is that as children who were abused, my brother and I did not want to believe that our Nfamily was evil---sure, our folks were very punitive, but it was always cloaked in "we only have your best interests at heart." I guess that it just got to the point were my brother and I were brainwashed---a child never wants to believe that his/her parents are bad or(worse yet) pathological. Maybe your mom does not see your brother for the psychopath that he is, maybe she continues to project upon him a false image of how she would really like him to be. It's kind of the polar opposite of what my parents projected on to me---I was the "bad" child(and boy, when you hear that often enough it's hard to shake).
Furthermore, was your brother was indulged in as a child?(I apologize if you already addressed that in an earlier thread--my head is spinning from all of the posts and the vast amount of information on this site!). In my experience, if a child repeatedly engages in behavior that is emotionally destructive to others and it goes unchecked, the child has the potential to grow-up into a monster(as is the case with my youngest Nsister---I do not advocate physical violence, but over the years my brother and I have joked in passing that what our Nsister needed was a good, swift kick in the pants).
