Stupid movie? Probably, but a silver tongue can go a long, long way. Like I said, no game. They couldn't sell it to someone who wasn't buying.
In the movie, one of the crashers tells the father of the bride or groom, that he is the son of the man's DEAD sister. And no, I don't know if that was bought, because that's when that clip ended. But it is far more than just INPLAUSABLE, for a man to not know the children of his sister and whether or not they were invited.
Any girl, who is bamboozled by a line, was looking to to be bamboozled. It has little to do with the guy and his "game".