Why are older sisters mean?

Because when they're feeling down they need to make fun of someone to feel good about themselves. They like the feeling as if they're in a higher position than someone because it makes them feel good. They feel as if they could just boss younger sisters because they're older. They like seeing that they could be in power of someone. But if you fight back and let them know you can stand your grounds they also will have a hard time trying to bring you down. Show them that you're strong and will not let them empower you.