plausible human sexual behavior

[anthrodisiac]

It does. In the second version, you need some additional precipitating incident at:

to force reveal of this unknown family relationship without the DNA test. That's an extra contrivance in the form of independent event, so it does, in fact, make the second variant more unlikely.

I mean, fair point. In order to find out they're related without the test, you do have to have both of them submit to a genetic heritage screening. But that could be as easy as, "I think I want to find out my heritage, in case I don't make it." "You know, I always wanted to do that. I'll do it, too! Solidarity, bro!"

Which, admitted, is fairly contrived and... shut up I hate you

Damn your... logic... Spock!

You win this round, Lobster...