I got into a fight for my step sister. . ?

Hey, Im 17 and just recently, my mum got married and now i have a step sister. I have no other sisters but i have a brother. I don't want to sound mean or anything but i never made a brotherly connection. Like i do talk to her and treat her with all respect but never actually showed her that i love her (family way lol) until today in school (i think). I was sitting in lunch hall and i noticed her in the que and someone pushed her and called her some names and i just went over and grabbed the guy and started fighting with him. I don't why but it was just like instinctive. It was weird, why did that happen ?