That could be a part of it, especially if she is perceiving feelings of people in her family being sick that may be dying and leaving her. Family is incredibly important to young girls and the thought of it changing, or them being left all alone by people they love dying is profoundly frightening for them. Being scared to sleep by herself is a suggestion of that fear of not being safe in the warmth of the family she knows if it were to somehow not be there anymore.