I hope Billy is in RE5...
Eitherway, though, I had a thought....
In Code Veronica, Wesker mentions taking Steve's body which held the T-Veronica virus in it. If that is Sherry (Even if it isn't, Oh well), why would she be in a test tube. All speculation, yes, but could it be possible that she was injected with the T-Veronica virus and is being forced to go through the whole test tube experience Alexia went through?
I don't know...but it's a cool enough thought, I think...