I once heard an experienced pastor say something like this: "You'll never understand alcohol as a problem until you understand it as a solution." In other words, for many people alcohol becomes a way to "solve" their problems, by covering them up and escaping from them. But that never works, because the problems still remain -- and alcohol only makes them worse.
From what you say, I suspect your father feels the pain of your mother's death very deeply, and he's using alcohol to escape from that pain. Not only does he miss her, but he also probably worries about the future and how he'll manage without her. At the same time, he may not even realize what he's doing.
There's no shortcut; you and your brothers need to confront him with the reality of what he's doing. Let him know -- openly and clearly -- that you love him, and because you love him you can't bear to see him destroy himself. Remind him also that your mother loved him, and the last thing she would have wanted was for him to become enslaved by alcohol. Find out, as well, what resources may be available in your community to help him; your pastor or family doctor may have suggestions.
Assure him, too, that you're praying for him -- not just to break free from alcohol's grip, but to turn his life over to Christ. May the Psalmist's prayer become his: "Why, my soul, are you downcast?... Put your hope in God" (Psalm 43:5).