Is a divorced stepmother considered a forbidden degree of kinship (mahram) to the questioner's brother, even if he continues to communicate with and visit her despite her inappropriate behavior, immodest dress, and the commission of prohibited acts by her?

A stepmother is permanently forbidden (for marriage), and her ruling is that of maharim (unmarriageable kin) regarding the permissibility of looking at her, shaking hands with her, and being alone with her. It is permissible for her to show her head, neck, palms, and feet in front of her husband's sons, unless it is with lust or fear of temptation. If there is fear of temptation or intention of lust, then even a mere glance is not permissible. Taking pleasure in maharim is a greater حرمة (prohibition) and جرم (sin), and it goes against natural inclination (fitra). If you see any suspicion from your brother towards his stepmother, advise him to be mindful of Allah. If he persists, then inform your father.