In the kitchen, choose to not let Sasha out.
Viewing post in Voronica Cleans House: a Vore Adventure comments
Assuming that you are on the current, 1.02 version I don't know what the problem is (this problem has come up before and was fixed previously - back then the achievement was still achievable on the full breast expansion line, so you can try that?). I've checked the code and everything does seem to be in place.
Also, just in case there has been confusion, if by chance you mean Ending 3, that is done by equally mixing breast expansion and giantess transformations.
Edit: Actually, are you on mac? Looking at my files the fix may not have been uploaded for Mac.