Ava Parulis is described as a Gold Digger. When we later learn that another sister was being forced into prostitution to pay off a debt, it seems likely that she was like this in order to get the money to buy her sister's freedom.
Logic
George Burton is stunned to learn that his wife is dead and that her twin sister has taken over. Fair enough, except. . .you didn't notice ANY changes in her? Her personality? Her domestic skills? Her bedroom skills? That she suddenly didn't know the names of friends, neighbors, etc.?
It's a combination of Genre Blindness and Good Cannot Comprehend Evil. Dead identical twin impersonation is just not a thought that would ever occur to a normal person.
Apart from the factual details (which Ava presumably used some kind of strategy to piece things together without tipping her hand), don't forget that Irina was a mail-order bride too. How well did George really know her by that point?