Should I plead guilty, no contest, or not guilty?

I'm being charged with simple battery, my arraignment is tomorrow but I'm not sure what I should plead. A large 19 yr old guy (more than 300 lbs) charged at my pregnant and underage sister (150 lbs), I stepped in front of her and he shoved me so I shoved him back and slapped him twice. We left because my sister was terrified and the cops didn't get our statements before the state attorneys office charged her. What should I plead to establish just cause? I believe they will drop the charges once they hear her side of the story. Help!