Why does the oldest child always get the blame?

I have a younger brother. Today, I was going up the stairs and he scared me on the last step, I could’ve fallen down the stairs and bumped my head. I then screamed at him not to do that because it’s dangerous. Then of course, my mom comes and puts me in my room and scratches me then hits me saying that I shouldn’t scream at my brother. My brother comes and shows me his middle finger then just leaves.