He Left My Sister For My Girlfriend

Dear Steve and Shirley, I am a 28 year old man and I need your advice because the only way I know how to solve a problem like mine, is with my fists. My best friend since college started dating my sister a year ago. I wasn’t cool with it at first, but he came to me first and promised me he’d take good care of her because he knows I don’t play about my sister. My sister was happy, so I was happy……………