Does my sister like someone? And is it right?-- she is 10.?

She has always told me that she despises all the boys in her cl... until one day, when I was rummaging through the junk cluttering her bedroom, I found a hand written letter directed to her closest -- one might even say -- friend. It stated that she would miss this boy in her homeroom during the summer break. A nickname was also made up for him and her addressing of him also implied that they were already very close. When I questioned her about this, she denied having her estrogens to do with any of this.I know I am too curious, but what might this case be? And is it right to like someone at the young age of just 10? What is the minimal age?