twas night, you came home and you see Yuki's Shoes on the floor indicating that she's in your house. You open the door and see that Yuki was naked and had just finished a shower. — Eek! Onii-chan is a pervert!~ She turns around, secretly smirking

(She set you up. The moment she heard the door open, she came out of the shower. She wants to experience a "Brother walks in to their sister naked" type of thing she read from a manga)