How do i forgive my mom for what she's done to my father?

Obviously your mother needed more from your dad, and she didn't get it. Hopefully you never go through that. Don't take sides, she didn't do this to hurt you intentionally, people change as they grow older, and grow apart. It's nothing against you or your family. Your mother has her reasons. Maybe the smoking, complaining, and so much time on the computer, not paying attention to her, wore her down. She won't last with this guy. But she has to make her own mistakes, just like you do. Your brother cannot blame his alcoholism on your mother's affair. Remember how you felt when you first fell in love? How giddy and excited you felt? After many years, if you don't keep this alive, love dies.