You're pretty naive if you think it's not a possibility the girl isn't telling the truth.
And kids DO lie, even about important stuff. Even kids from "good families." Stranger things have happened. If she is lying, more than likely she was put up to it by her older sister.
I'm not the only person who thinks many aspects of the story are fishy. It's certainly quite possible the girl is telling the truth and her sister was abducted by a stranger, and it's certainly wise to operate under that assumption, particularly for the police.