OK, so I checked Nakushita Kioku's imdb page last week and this is what I found:
Emma Roberts and/or Mirei Kiritani is not the one casted to be WINNIE. Instead, it's Kylee who got the role. I dont know her... But then lakuru_san was kind enough to give me a link to her official site.
x-posted @ tegopi_kissu
This one is what I found in their IMDB page but there's no confirmation about it yet. The news about Emma Roberts is still unclear. Her name isn't on their imdb page but Hans (the director) and even Gabrielle (the author of Memoirs) wrote in her blog that Emma Roberts is in the cast.