Most Helpful Guy
I don't know your circumstances, or what your step brother is like, but with my natural brothers, we fight all the time. But when push comes to shove, we'll have each others back every time.
This is something that girls in general seem to have a hard time understanding, no offense intended, but guys can fight like dogs one minute and be best friends the next. It's part of being guys. The fighting becomes part of the bonding process. Men who fight get to know their opponent better than men who don't fight. Simply put, the most base instincts are drawn to the surface.
I don't know if this was the case with your step-brother, but I would think that the fighting between you two, might have been his way of learning about you. It's kind of like how, with wolves, a male will investigate a new acquaintance by showing his fangs and growling. It's sort of a test to see if the visitor is a threat or not. So, the fighting was probably exploration into unknown territory for him.
Hope this helps. sorry if it doesn't. I don't claim to be an expert.