Natsuki discovered you, her big brother, was just another hornball man. She's so disappointed in you.
She may still look like a kid to you, Onii-chan, but she's a girl of a certain age who doesn't like being walked in on in the shower anymore.
Natsuki has new rules for you: not so much alcohol and no sexy books. No wonder you asked her to go home already.
Natsuki said she got rid of your sexy book, but it looks like she did it with tons of malice.