Jewish nonsense from the Kabbalah. It's a branch of mysticism that really attracts psueds because Alister Crowley talked about it once, and Crowley is someone I find fucking hilarious because the minute someone starts talking about that man you can kinda write them off depending on if they talk about him as a spiritual leader or a businessman. Hulk is a branch of the Jewish tree of life or some shit, he's there to clean up messes, blah blah blah dualism blah blah blah.
I prefer where evil doesn't actually have mass of it's own. It's a parasite or corrupted good that's born out of sin. If you go the Divine route then Good doesn't understand Evil initially, I hate niggas who go dualism like the Devil is God's equal when the Devil's job in the majority of Christianity, Islam, and Judaism is to tempt, if you fall it's on you. So yeah, Banner and Leader being related is so we can have more basic bitch dualism.