If I were the twin sister there is no way I would be fooled by that geezer mainly because after what they have seen so far, they should assume everyone wants to belittle them and hurt them and only trust people after they prove otherwise. It's a sad concept but it saves them the heatache and the physical hurt.
But I suppose it's exactly that desperation that made them blind to this guy. Wanting just one person to accept your existence and meeting someone who seemingly does so.... I suppose it can blind a person to others' malicious intentions.