Onii-san made the classic blunder: leaving the girl you like alone in your apartment as she tidies up.
Ai-chan suffered the classic jealousy without a target since Onii-san was basically fantasizing about her in a waitress uniform.
Ai-chan didn't want to believe her waitress uniform was sexy. It's cute! Only cute! Oops, she broke stomped on one of his disks during her rant.
Ai-chan felt bad about breaking one of his cosplay disks, so she offered to "replace" it with pictures of her own self!
There was some heavily implied hanky-panky going on here. How do you take gravure shots with the lights turned off? Unfortunately for Ai-chan and Onii-san, a deliveryman interrupted their cosplay shoot.