I doubt this is stuttering, but it does sound like your sister has some type of speech impediment keeping her from speaking properly. As others pointed out, a lot of people have these as children and grow out of them as she gets older. Depending on the level of severity, the school might step in and put her in speech classes, which might be a great thing for her. If no one steps in and it continues, she might want to see a speech therapist who specializes in kids.