How can I help my little brother?

Here's the thing: my little brother is in elementary school (2nd grade) and very spoiled. Ever since birth, my grandma has given him everything he's ever wanted and my parents give into him because they don't want to deal with his yelling. As a result of his spoiling, he behaves however he wants, frequently saying he'll kill us if he doesn't get what he wants, treating women like crap because he believes they should serve him, overeating in a vain attempt so that no one else will get any food, cursing on a regular basis, never doing anything to help around the house or be considerate, even when told, and frequently spitting/hitting/kicking people for no reason. He never has any exercise and has gained twenty pounds in the last year, is now overweight, and never eats anything remotely decent. My parents are well aware of his behavior and blame my grandma because she mainly took care of us for the first six years of his life but it's also my parents' inaction. They never punish him other than yelling and then giving up. They're not bad people, they just work so much that they don't feel up to full-scale parenting. As his big sibling, I'm really worried about him. I don't want him to end up like my uncle, a dependent leech who's been to jail three times. I try to discipline him and limit him but I usually get yelled at by my parents because "I'm not the parent in this situation." I'm just so frustrated, and scared, and angry, and I can't figure out what I can do. I've tried talking to my parents and talking to him and none of it works! I love my baby brother so much and he really does have a good heart and it kills me inside because he's so awful and he's on the fast track to CrappyLifeVille. What can I do to help him get better?