Hey Es,
Tell them you do know better. You know better than to marry some stupid brother who's only reason for getting married is to finally have sex. You know better than to stay with a religion that says it speaks for God, but has hundreds of man made rules. And you know better than to give your life to a religion that is doomed.
Don't worry about them not coming. My wife and I had a wonderful wedding without my family. Funy thing is my sister couldn't figure out why we didn't come to her wedding. I mean, duh, you don't come to my wedding, I'm not coming to yours. They just don't get it do they?