What should I do about my father watching pornography, and should I tell my mother and younger brother or confront him?

You must condemn your father's bad actions by advising him and reminding him of the impermissibility of what he is doing, using a good approach. If he does not respond, seek help from those who can influence him, such as people of knowledge and righteousness. Do not tell your younger brother what your father is doing, as this is slander and impermissible unless he openly commits the sin. You can seek help from your mother if she is wise and capable of reforming him. Continue to honor your father, for honoring parents is obligatory even if they are disobedient.