My little sister is not the most popular girl at school?

It could be that those kids are jealous of your sister, so they use her than leave her. In this generation and age, younger kids are getting more and more concerned about looks and styles. So because your sister is pretty people want to take advantage of that. I also had trouble making friends, people used me and left me. I was a depressed child, and I still am, though not as much. Tell her, to be very friendly(I have no doubt your sister is friendly) but like just tell her to introduce her self. From there try to find common interests, which should start a big spark in conversation. When your sister sees that person tell her to say Hi all the time. People love it when they are recognized and complimented. But don't do it where it will get too annoying. Tell your sister not to get down. I am similar to your sister. My brother was very popular in school so he left a reputation. So when I went to High school and he went to college, a lot of people expected a lot from me. I was just not that kind of person. I had hard time making friends and I had trust issues too. Tell her to be positive, don't be down. Tell her to stand up for herself. If she doesn't people are going to continuously step over her, and it's going to form a reputation that will last a long time. She can maybe say "Look, I don't understand why you just used me. Is there something wrong. Did I do something wrong? All I want is to be friends." etc etc. And go on from there. I hope that helped!