If he was sharper than anybody, Amycus was sharper than his sister. He could see that she was coming across as defensive, which would only arouse further suspicion from their Death Eater friends, and she also seemed to on the verge of starting a fight, which would draw mass attention to them and perhaps result in their mischief being discovered.
"There are lots of different types of books, Avery," Amycus said, keeping his voice calm and laying a hand on his sister's arm. "You can't lay claim to them all. Alecto and I have been browsing happily in the pornography section."