I agree it's a little sloppy execution of the twist, but it's not immersion-breaking. Nobody told because everyone assumed the Mc knew, it was common knowledge. I can relate to that. Let me explain:
I come from a family of teachers. My dad was a teacher in my junior high school and my maternal uncle in my high school. From day one almost everybody knew who I was related to. I never bothered telling any new friends I made, because I assumed they knew already. It was a given, I never thought about it. So Sage or anyone else not telling Fuckface is something plausible, to me at least.
Now, the Mc not knowing the last name of a girl he is fuckbuddies with or romantically interested in? That's harder to swallow. Maybe he knew but thought there was no relation? I'm not a native English speaker but I believe Burke is a quite common surname in the English-speaking world, or at least not rare. And we are shown that Fuckface is a little boneheaded sometimes.
Like I said... sloppy, but not outright implausible
