I believe its sherry, its too much of a coincidence, especially after looking at wesker's report. If you remember playing through RE2, i think the canon senario is probably the one when sherry is injected a g-virus egg. Wesker himself says "I did not underestimate birkin, but thers something about this girl"
However, what im bothered is how will she react when she hears the name Redfield. I mean, i remember shery saying that she hopes Claire wont forget her.
As for jill, i dont think that its her. Although some theories that have been said before such as the similarites in facial expressions, i think there is more evidence proving that it is sherry. Even the producer (i think) has said that sherry is in the game. But in the end, its still not entirely decided if its sherry or not.
i talk to much...lmao