Now we’ve got the answer to one of those questions. When a fan asked PLL writer Andy Reaser on Twitter if it was possible for the ghost to be a young CeCe, Andy replied, “I don't see how... it would involve a serious disruption in the spacetime continuum.”
So the girl is definitely not CeCe, which leaves us with it being either a young Ali, Ali’s twin or someone else altogether. Though the girl being Ali or Ali's twin would also be a space/time disruption ... unless it really is a ghost! All we know is that she's definitely someone with a sister, which eliminates a lot of the known members of Rosewood.
If it was a vision of Ali (or her twin), why come to Ashley? Is it because she’s the only parent around Rosewood, or is that another clue?