She’s still at a pretty young age, so you can wait to see if she’ll “grow out of it” first. Some stutters can be common for shy kids, especially when talking to adults. It’s best to ask your sister some questions how she feels from her stuttering. Does she feel self-conscious or anxious by her stutters? Are there any specific words she feels that it’s easier or harder to say? Does she dread having to talk from her stutter? Does she feel like there’s something “clogging” her throat from trying to get the words out? If yes to most of these sort of questions, then it’s likely a speech impediment and she should seek help from speech therapy.