I don't want to be the Best Man and I Think the Groom is unfit

Please help. I don't know what to do. You see, my sister is getting married this year. For some reason,I, her younger brother, have been selected to be the best man. However, I cannot stand her fiance. I have my reasons. Firstly, and most aggregously, he is lazy. He does not want to work, he does not want to learn, and he always has to be right. This is the kind of a guy who will lecture when you show the slightest sense of ignorance to a subject. It's like pulling teeth for my sister to get him to do chores around the house when I am over. I can recall him asking me to clean after his pet. He's been fired before for these very reasons. In the months that he was unemployed, my sister was stressed. Her voice gave it away. My parents, and even my sister, have admitted he is lazy. An indicator of his sloth is his weight. He is nearly 300 lbs. Whenever my sister, him, and I go for a walk, he has to take many breaks. Moreover, his higene is abysmal. I can recall several times he's been to my apartment, smelling like shit, and then falling asleep on my sofa or bed. It's shameful. The worst thing to me, is his negativity and disregard for other people's perspectives. He would have you believe he is a tolerant man, but lord help you if you disagree with him on anything! As I mentioned before, he will lecture you, talking down to you if you disagree with him or give him the idea that he knows something you don't. It's maddening, and I can't stand the idea of calling this slob my "brother-in-law".

I know it's ironic because I'm going on a rant myself, but this is because I want what is best for my sister. My tolerance has nearly waned and I do not want to be the best man. I do not see myself as his friend. What can I do? Of course I have never told this guy I don't like him, because that would be rude. And for a while,I thought I could withstand it. Well, now I can't. He's just plane unfit. What would you do?