Is it permissible to say to one's father, "I disavow you, and Allah will not pardon you," because he wishes to marry the maternal uncle's wife after the uncle's death, especially given a prior relationship between the father and the maternal uncle's daughter?

A Muslim is not permitted to address his father with words that displease him, because Allah has commanded companionship with parents in a good manner. Although a father's marriage is not considered an offense to the mother, if the relationship between the father and the cousin's daughter has reached the point of zina (fornication/adultery), then he is not permitted to marry her mother. If the father establishes a forbidden relationship, even if it does not reach the point of zina, then the children must advise him with politeness and gentleness. The children must honor their father and obey him in what is good, for honoring one's father is among the best acts of drawing closer to Allah.