Should I obscure the images of unveiled women in textbooks, knowing that my brother will get angry and verbally abuse me, and so will my mother, if I do?

You are not obligated to erase the forbidden pictures in your brother's book. Your duty is fulfilled by advising him and clarifying the Shariah ruling to him, based on the Prophet's (peace and blessings be upon him) saying: "Whoever among you sees an evil, let him change it with his hand; if he cannot, then with his tongue; if he cannot, then with his heart—and that is the weakest of faith."