What should the questioner do with her brother, who was formerly addicted and whom she suspects has relapsed, given his refusal to talk, her fear of him for herself, and their father's complete acceptance of him?

If the brother repents with an apparent repentance, he should not be suspected of evil unless there is evidence, and the default is to presume the innocence of Muslims. If his family becomes certain of his return to addiction, they must denounce it according to their ability: secretly if he is concealing it, and openly if he is manifesting evil deeds. This may include shunning him to deter him, as long as it does not result in a greater harm. Praying for his righteousness is a legitimate and recommended act.