Is it permissible to inform the elder brother about the father watching pornography and to discuss this with the mother, knowing that the mother is aware and has no objection?

You must conceal what your father does and not inform your siblings or others, while gently and kindly advising him and warning him of the consequences of sin, and reminding your mother of her duty to advise her husband. It is permissible to inform your siblings if their help in changing the evil is hoped for.