Is it obligatory or recommended to object to a non-mahram paternal uncle residing in the family home for the third time after two previous attempts, and does the failure to object result in falling under a curse and legal penalties?

The fundamental principle is that denouncing evil is obligatory, based on the Prophet's (peace be upon him) saying: "Whoever among you sees an evil, let him change it with his hand; if he is unable, then with his tongue; if he is unable, then with his heart—and that is the weakest of faith." This is a communal obligation (fard kifayah). If the evil recurs, then denunciation must be repeated as long as there is hope of changing it, while considering one's capacity and ability, and not limiting it to a specific number of times.