This chapter felt like the author was bending the whole world to make the MC oblivious to the fact that the mysterious guy was her brother. It's amazing that he didn't realize it was her brother when the guy was this familiar with her, was exactly one year below her and looked pretty much exactly like her.
And even if he didn't realize despite all of this, wouldn't she tell him after he referred to him as "that guy" on page 13? That seems like a natural course of action.
The whole chapter filled me with second-hand embarrassment, but page 6 was pure cringe. Pookie was too jealous to listen to the explanation, so he had to make a scene and run away. What a joke (but how convenient for the plot, right?)