How do I Win Against a Dirty Fighter?

My twin brother fights really really dirty. Fingernails, punches to the balls, even biting, nothing is off limits for him. Its hard to win against a guy like that. And he has no respect for honor or anything. When I have him in a hold and I can see that he's in pain, I let him go, because I'm a nice guy, but as soon as he's free, he attacks me. I took karate for 7 years, but if I beat the living **** out of him, he would go probably go psycho and cause permanent harm to me, or my belongings. He's legitimately crazy, I'm not exaggerating. I'm 16 and about 5"3" and about 120 pounds. Hes a little taller and a little skinnier. And the other thing is that he never showers, so he smells terrible, so whenever I fight him, I feel like vomiting because of the smell. I've been putting up with his bullshit for my whole life, and I'm done with it. What should I do?