It's the whole smugness factor involved. Like the forces of fate were like, "Nyeh, I bet you thought you know everything in advance. How do you like THIS being under your nose the whole time?" And making rude gestures, of course.

Kevin is the eldest of his fam, probably around 16ish [just the right age to be fatally mortified by the parental(s)] whilst Trash is... a different story. Goblins are "considered mature" at twelve, so she could be an official adult whilst also being younger than Our Kev.

There is for sure going to be some sibling rivalry.