Jealousy

This girl I dont like is talking to my boyfriend all the time. I hate her and he doesnt even care that it makes me mad. He goes to the extent to say that shes like a sister to him when they've barley reknown each other for a semester of school. They wemt to elementary and middle school together but she's a grade under him so its not like they've ever had classes together. Im so pissed off and Im jealous of her because shes pretty and crap and Im scared shes trying to take him from me. What should I do??