Does thinking about giving my brother a magazine that contains pictures of unveiled women—knowing that this is forbidden—constitute making the unlawful lawful?

Your question does not involve making lawful something that is unlawful. Rather, you are suffering from whisperings (waswasah) related to thoughts of disbelief (kufr), and the cure for this is to ignore them. We reiterate our advice to ignore whisperings, as this is the best remedy after seeking Allah's help. As for the ruling on possessing beneficial magazines that contain pictures of unveiled women, there is a detailed explanation that can be found in Fatwa no. 63607.