Although common sense would scream
fix it, for your own self if not the grandmother, I caution you to be aware that you could get serious backlash from the grandmother if she doesn't really want to change things ~ then you are tagged the bad guy, not the brother. Some people are very entrenched in behaviors and relationships that seem ridiculous and destructive, and they may complain long and loud... but really don't want to do anything differently.
You can't make your brother respect your grandmother, and you can't make your grandmother demand he show some respect. At best, you can demand that YOU be treated with respect.
You may have grounds (and it sounds like you do) to intervene and report elder abuse, but just be aware that it could stir up a whole different hornet's nest.
What If I Suspect Abuse, Neglect, or Exploitation?