From chapter 134, part 2

So that "Tomo" guy mentioned that he had an older sister..

Heh..

It's a bother, I better stay quiet about this.

And he did say something like "Haven't you met her already?"...

Hey Kuroki, what was your name again?

To...... Tomoko

So you're also a Tomo? What a coincidence!

Hehe...

Delinquents really aren't that bright, huh.