You might also talk with her sister and the grandmother -- again in an off-hand manner. I suggest that you not reveal why you are asking questions up front. You also might want to consider whether anything happened to you -- were you in any danger, were you sad -- while she was away. She may be afraid of losing you or be afraid for you.
After you find what is frightening her, you will know how to proceed. If unsure, consult with a pediatrician or a licensed professional counselor.