By climbing your family tree back up until you find out who's your Jewish ancestor.
I think the troper means, how do you have an odd integer fraction.
Jack and Dina had a son in his late teens/early twenties in the first film who played an important role in the misunderstandings, but is nowhere to be seen in the sequels. The third film involves an heir to the Burnes name and he's not mentioned at all. Did he die? Did he and Jack have a falling out due to his pot smoking?
Considering how much of an asshole Jack is, my guess is that the son got sick of his shit and stopped speaking to him.
It was Denny.
Why is the rest of the Byrnes family so harsh to Greg? It kind of makes sense that Jack wouldn't trust him, Papa Wolf and everything, but the entire family is pretty rude to him the moment they meet him.
They're probably afraid they'll end up on Jack's bad side, if they're seen being nice to someone Jack has it in for.
Plain and simple, he didn't fit in. He was Jewish, a male nurse, and pretty mediocre-seeming, and when they had seen perfection (in the form of Kevin), it was hard for them to accept Pam dating anything less.
Why did Pam never do anything to stop her family from being rude ot Greg or try to stop Greg from continually lying? The moment she put her foot down at the end, Jack let off. Why not do that earlier? Granted, the movie would've been shorter but they could have hand wvaed it somehow by having Pam be gone for most of it. Instead, she comes off as pretty dense or at least apathetic that her loved ones are being so cruel to one another.