Genevieve, 

I am a step father having raised my late wife's son from age 6.  He is now 32 and he treated his mother terribly the last 6 months of her life.  In fact his last words to his mother were "F-off, don't ever call here again".  I know he suffers from unbearable guilt, but I told him the day before the funeral his mother loved him no matter what and though he could not make amends to her by apologizing, he could by changing the way he treated his wife and boys.

Also I know had it been me that had died instead of his mother he would be going through the same emotions as you.  Here is the good part, this man knew how much you cared.  You may not think so, but I'm pretty much in the same position as he was.  My step son had anger issues over his bio father and always took it out on me.  But I knew he loved me and more important, he knew and knows I love him unconditionally.  Don't carry a burden that may not exist.  As for how your mom treated him, that is not your fault or responsibility.  Don't take on her guilt.  If you have sine abger over how she treated him, seek some support, group, or one on one and work through it.  If you need to tell her how you feel, you have to do it in love, not anger.  You are in my prayers

Jim