Alaric was confused,
'But Tia does not like meeting with the other aristocrat children.'
He knew his sister did not really like to gather with the aristocrat children, so that sentence was a lie. But, she was his sister and the cousin sister to Ivan, Beatrix and Serina, so saying that she wasn't was also a lie.
Maybe his sister was starting to like meeting with the aristocrat children. That must be it! Alaric thought, feeling like he had suddenly figured it out. That meant this was the truth, and the one about her not being his sister was indeed the lie.
Aristia didn't say anything, just smiling at their guesses. She was telling the truth, after all, she wasn't really their sister and cousin. The last one was a lie. Heck, she didn't enjoy meeting with those aristocrat brats at all.
"Which one is it?" Beatrix asked.
"You all... guessed correctly." She said with a small smile.
