Is it right to hate one's cousins...?

I'm confused, confused about whether it is right to hate my own cousins. A lot of people, including my friends at school, asked me to let go the hatred I harbored ages ago towards my cousins. I tried but I just can't! I just cannot let it go! I wish to, but I can't! Everything always went back to the beginning. Like one time, when I was back from school, I saw my brother on the ground, crying. It was obvious that my cousins were at guilt but they denied it. I forcefully extracted information from them and the truth was they were the bad guys. But my parents scolded for being rough on them. They pushed me and my brother to the extreme but we couldn't do anything except to get toyed by them! The feeling, the embarrassment, the humiliation was terrible! Although I was the older, smarter, taller and tougher one, I couldn't do anything! The hatred, the grudge, the abhorrence, the detestation, it's just UNACCEPTABLE! But as the older and the more mature one, I just can't stop thinking that it is wrong to hate my cousins.... What should I do?