i hope never. there's no reason to. all it would do is dredge up bad memories from their past. Though, the fact that they call her the female version of Henry, does tend to indicate that Peter does tell her at some point. Then again, it's quite possible that Olivia could remember the cab driver on her own and remember what he did for her and decide to call the baby the female version of his name. But if i had to pick a point in time when he told her, i guess i would go with after they had Etta. Then maybe she wouldn't be hurt as much by Peter having a child with the fake her.