It will never actually be known which sister it was, sadly. I can only tell you that Bob Atchison was given the privilege by Dr. Avdonin to actually see the original remains found, and Bob was convinced from the first moment that it was Anastasia, not Maria and thus Maria was the "missing" girl found with Alexei. Now, the Russians had no influence on his decision, and why is this of any meaning? Well, Bob was trained to be a professional portrait artist, and studied under Chuck Close (google him), and he is of course utterly familiar with what each member of the Imperial Family looked like. However, it is, in the end, just his opinion.