Your mother will have to ask the court to grant her custody. Although it is not the Cabinet's decision, their opinions usually hold a lot of weight with the Court. Your mother's best chance is to demonstrate she has made the necessary changes to get back on the right track with her life and, absolutely, is not using drugs. The Cabinet has probably already given her instructions on how to do that. Even so, the Court will make a decision it feels is in your and your brother's best interests.
As for your grandmother, if you feel her behavior has reached a point that would affect your well-being, then you should discuss the matter with your grandfather, as well as other adults, such as your school guidance counselor, that can assist you.
