What should the family do with a father who was once loving but then came to hate them, watches pornography, knows women he spends money on, and forsakes his wife, while it is said that he is a mosque imam and known for his religiosity and piety?

You should supplicate frequently for your father's guidance, advise him kindly, and seek help from those who have influence over him, as this is part of the obligatory benevolence towards a father. It is not permissible to describe your father with ugly words such as "he deals with us with complete hypocrisy," because that is an act of disobedience (to parents). As for financial support, it is the husband's duty to provide for his wife and young children who have no money, with the necessary food, drink, housing, and clothing. Regarding the brother, he did not make a mistake in advising the girl to stay away from the father, though it would be better for the advice to come through one of her female Mahrams. The father's abandonment of your brother is not permissible. You should try to understand the reasons why the father's love turned into hatred and address them if the cause was a shortcoming on the part of the children or a misunderstanding.