Is a child considered undutiful to his parents if he forbids them from lying and slandering people, and they become angry with him and curse him with their displeasure?

Denouncing the injustice perpetrated by parents against some relatives is not an act of disobedience (عقوق), but rather an act of righteousness (برّ). Their anger or curses against you merely for your denunciation will not harm you if you are righteous, for a parent's curse upon their child is effective only if the child has wronged them. The denunciation against them must be done gently and respectfully, without harshness or violence, because parents are not like others when it comes to enjoining good and forbidding evil.