How can one act concerning a father who molests his daughter and prevents her from marrying by fabricating flimsy excuses, knowing that the mother and uncle are aware of the matter but cannot confront him?

It is not permissible to remain silent about this great evil. What is obligatory is to refer the matter to whoever has the ability to deter this tyrant. She must refer her case to the judiciary and claim that he is preventing her from marrying (he is an 'adil' - one who withholds a woman from marriage). If it becomes necessary to expose his affair before the judge, there is no impediment; rather, it is obligatory if she believes he will revert to his ugly act.