Your letter caused me great sadness and not just because of the abuse your family has endured all these years. I was also saddened as I thought of your father, because he has obviously been unhappy and discontented all his life. How sad and how different his life (and yours) would have been if he had faced his sin and found the peace God wants to give us.
Now he is facing eternity, unprepared for death and without hope and that's one reason why you shouldn't abandon him. He thinks his greatest need is to have everyone meet his immediate desires but in reality, his greatest need is to prepare for eternity by giving his life to Jesus Christ. Only Christ can forgive his past, and only Christ can change his attitude right now. Most of all, only Christ can give him hope for the future.
Pray for your father, that he will realize his spiritual poverty and open his heart and life to Jesus Christ. Pray also for your brothers and yourself, that you will show Christ's love to him and also have the courage to confront him with the Gospel. Never forget, God loves your father, just as He loves you. The Bible says, "Do I take any pleasure in the death of the wicked? declares the Sovereign Lord. Rather, am I not pleased when they turn from their ways and live?" (Ezekiel 18:23).