Is it permissible for me to say "Allah (alone) is sufficient for me, and how excellent an agent is He" (Hasbi Allahu wa ni'ma al-wakeel) regarding my father who frequently curses and invokes evil upon us, especially my mother, and neglects us? And what should be done in light of this familial estrangement?

The right of a father over his child is immense and is not forfeited by his injustice or ill-treatment. The saying "Hasbiyallahu wa Ni'mal Wakeel" (Allah is sufficient for me, and He is the best Disposer of affairs) might be considered a supplication and an offense in common understanding. Therefore, one must apologize to the father, seek his forgiveness, and be diligent in honoring him. There is no objection to advising him gently and respectfully, especially concerning his ill-treatment of the mother, for enjoining good and forbidding evil upon parents differs from doing so with others. Similarly, one must enjoin good and forbid evil upon siblings and encourage them to honor their parents.