25 stayin w/mom & bro is 17. If he antagonizes me to the point of us fighting....what happens legally?

Wow, wtf is wrong with him? Talk to a family counselor. Hell man, move out of the house and dont even think of including him in your life. Block that fvcker out, and go on, make your money, meet a nice girl (or guy, whatever floats), be kind, donate, buy a nice house, get a nice job, go to school and everything. Obviously your brother is fvcked in the brain, so when he comes crawling up to your door, begging, you throw him onto the street.