My brother is being really unfair, what should I do?

O.k this is kind of a weird situation, but anyway. One of my elder brothers is a science teacher at my school, and unfortunately I am in his cl for science. Yesterday in science, me and a group of friends were messing about and not getting much work done, Anyway, I we ended up knocking a bottle of diluted hydrochloric acid off the table and it smashed all over the floor. It was an accident, but I guess he was right in saying it wouldn't of happened if we hadn't of been messing around. He kept the 3 of us in at break, and made us clear it up. Then he put ME in an after school detention but not my two friends. I know what I did was wrong and everything, but still it wasn't just me. I wouldn't of minded if my two friends hadn't of got off scot free. He's treating me differently just because I'm his sister, I've mentioned it to him but he doesn't think he was unfair. It's weird enough for me as it is, without him picking on me. What should I do?